Typically given as glib advice to "Just re-install Windows, you won't lose your settings!". This is very bad advice unless a number of questions have been answered first, with a high degree of technical certainty. Following this advice as a generic first step can cause such a disaster (irretrievable data loss, unbootable PC) that you will wish you could return to your original problem state. There are far better general approaches to PC problems.
What does it do?
Re-installing Windows over the existing installation does the following:
When this works, it does so because:
Often it is (2) that fixes things - but you can see that (2) means "you won't lose your settings!" is only partially true. The more you deviate from duhfault settings, the more you stand to lose.
Hardware problems are the biggest risk. You start with working hardware, through which your original code base was installed. At some point, your hardware gets flaky and you start to have problems. At this point, you have a mainly intact code base; maybe 5% bad from secondary damage due to bad exits or wild writes corrupting the file system, as well as recent installations done through the lens of bad hardware.
So you "just re-install Windows" to wish these undiagnosed problems away. You now have a 90%-suspect code base, because the newly-installed code has been installed through the lens of bad hardware. The bad hardware can also blur the file system rules on where this material is written to, so there's risk of overwriting your data or trashing the file system.
Simply writing 50M or so of files to an at-risk hard drive or file system is dangerous enough, even before you consider the quality of this material when run as core code.
So before you "just re-install Windows", you must verify that your hardware is working reliably, as described in this "before you think" page I wrote in the Win9x days.
Snookering yourself is another risk, i.e. starting a destructive process before making sure you have everything you need to complete it, i.e.:
Every "over-old" or repair install can spiral out of control and force a scorched-earth reinstallation if things go wrong; hence the need for (2), (4), (8) and (9).
You also have to deal with software vandors that requires special procedures before their applications can be re-installed. For example, you may have to de-activate the existing installation by phone or via the Internet, or allow it to write information to your installation disks so that these will re-install (let's hope this doesn't kill the disk).
If you don't jump through these hoops before you nuke the existing Windows installation, you may not be able to do this later. Or you may find your installation disks don't work, the product code you have doesn't match the disk, the new installation can't use or import the old data, or the vandor you need to talk to has closed down or wants to force you to upgrade to a new product your system is too old to run and/or you can't afford.
Loss of patches can be a killer, leading to negative mileage when the intention was to brute-force your way over an active malware infection. Not only may the malware persist, but new attacks can be facilitated by loss of patches required to repair bad code, loss of protective non-default settings, and/or the restoration of risky functionalities.
Version soup is a likely consequence of re-installing Windows over an existing installation. You are going to change version relationships, and it's a toss-up whether this will resolve existing problems and/or generate new ones.
Original problem persists or recurs. This is almost inevitable, given the narrow scope of what re-installing Windows can fix. For example, malware may remain active, become active later, be restored with your "data", or re-infect the system in the same way as last time. Or bad hardware may slowly bit-rot your new installation to death, assuming it isn't stillborn to begin with.
Be very, very suspicious of any PC vendor or warranty support that tells you "everyone knows Windows is flaky and needs to be re-installed every now and then". This is outright nonsense, and implies one or more of the following:
Low free hard drive space kills the installation process. Low hard drive space will cause crashes and this increasing death-spiral:
Some processes, such as printing, can consume a lot of hard drive space and crash the PC, while most other tasks still have enough space to work - so it's not always obvious enough for the Windows install process to refuse to proceed. Maybe the installation will barf partway through and leave you with porridge, or it will complete but leave an installation with even less free space and thus more likely to crash.
Forensic clues are lost, so that the troubleshooting you eventually have to do, is now more difficult or impossible to do. "Just re-installing Windows" breaks several cardinal troubleshooting rules, i.e.:
Techs that "just re-install Windows" rather than troubleshoot are missing out on valuable learning and skill-building opportunities, as well as offering the client poor value. As a general rule for stand-alone PCs, the faster technicians resort to "just re-install" or "format and re-install", the more useless they are.
In the corporate world of multiple professionally-administered PCs, this rule applies less. When all crucial data is on the server, the workstation is meant to remain in original state, and facilities exist to re-image it with a minimum of effort, this may be time/cost-efficient management... much of the time.
Variation: Format and re-install
Obviously more destructive, this is the usual second-step of the "if it doesn't work, hit it harder" school of maintenance. The idea is that if an over-old re-install failed, then a bigger shotgun will succeed. The approach will fail if:
Variation: Version upgrade / downgrade
There are few things less likely to work than installing a new operating system over an older one that is not running properly. You just add a number of new reasons for things not to work, e.g. inheriting drivers and software that aren't compatible with the new OS, adding new chaos to version soup and so on.
That's assuming none of the problems described above apply; low hard drive space, flaky hardware, active malware etc., as well as assuming the PC's hardware specification is sufficient to run the new OS version properly.
Just about anything is a better idea than this.
In summary
Re-installing Windows is a potentially destructive step with wide-ranging implications. It's not a shortcut for users too clueless to troubleshoot the underlying problem, because the same skill set will also be inadequate to manage new problems that can result.
The scope of what a re-install does is unbounded, so even savvy users can expect to trip over unwanted changes and side-effects weeks or months later.
Thank you for asking that question! I've written three pages that attempt to answer it, from different perspectives; two in the Win9x era, and one more recently.
"Before you think" covers a standard preliminary approach that I recommend before going interactive with any nebulous problems. It can save you from a "death on the table" or other disasters that you won't be able to undo, and it also describes the typical failure patterns for each of the common broad-scope problems you are looking for.
"The Bad PC" is advice I wrote for a learner tech I knew, and it's orientated to self-preservation. A user once told me how he got techs to troubleshoot difficult problems for free; he'd buy some cheap add-on, ask the tech to install it, then blame all the problems as if they'd arisen as a consequence of the job. Cheaper than a troubleshooting labor bill, plus you get a nice add-on as part of the deal.
"PC Crisis" is a newer article that outlines my current approach to really sick, crucial, or at risk PCs. It's time-efficient in that it double-tracks two sets of tasks that often chew up "click and wait" clock time. I have yet to regret "wasting" time on this, but have already had cause to regret not escalating to this earlier, since I wrote the article.
Other articles cover approaches to DLL problems, formal virus scanning, what to do when NTFS makes maintenance difficult, and the Win9x front page has a section on FATxx data recovery. I want to write on commercial malware and a general approach to disinfecting PCs, but those tasks are daunting me at the moment.
(C) Chris Quirke, all rights reserved; July 2004