See my answer to last week's question:
[The title was "People are too quick to reinstall Windows "; and the first paragraph was:
People are too quick to reinstall Windows, and too slow (FAR too slow) to consider that when a computer isn't working, the problem might be the computer (and not the software).]
There is an old saying that when the only tool you have is a hammer, you tend to treat all problems like nails. This is kind of like that: Often people (in particular telephone tech support people) don't know what to do, so they give this advice. For THEM, it works, because "reinstalling windows" can take hours or even days, so the problem goes away, for hours or days. Note, however, that in their mind, the "PROBLEM" isn't the problem but rather the customer who is seeking assistance. Hey, if we just got rid of all of our customers, there would be no problems !! Think how much easier life would be.
So much for today's lesson on how to win friends and influence people.
Ok, let's look at VALID reasons to reinstall Windows:
1. Malware and Virus': If a system gets a virus/malware infection, reinstalling Windows may be the only way, the best way or the fastest way to fix the problem. These infections alter critical Windows files as well as installed user programs and, sometimes (but fortunately rarely) data. Some of them cannot be removed, in other cases hundreds or thousands of files may have been altered and a complete reinstallation may be the only way, the best way or the fastest way to fix the problem. IF, that is, the system is really suffering from such an infection.
2. Registry corruption and bloat: One of the most important components of Windows is the "Registry", a database of everything that is on the comptuer and that has been done to the computer. This includes both hardware and software. As the computer is used more and more, and as software and hardware are removed and added (especially added), the registry gets bigger and bigger and bigger, which makes using the computer slower and slower and slower (one of the problems is that when you remove a program, which may have thousands of entries in the registries, it's "uninstall" process probably does not remove all of it's registry entries). While there are "registry cleaners" that purport to "clean" the registry (and in some cases to defragment it as well), both their effectiveness and safety are variable and questionable; in the worst case, they remove critical entries and can damage or even destroy the computer (in some cases to the point that it no longer boots). Sometimes, you just get to a point where the best thing to do is start over clean, but the only way to do that is to reinstall everything (not just Windows, EVERYTHING) from scratch. Yes it happens. I'd even go so far as to say it's inevitable. But unless you are one of those people who installs WAY too much (too many things that you should know better than to install), it should take 2 to 3 years before you MAYBE get to a point where registry bloat is enough of an issue that you should consider reinstalling windows.
3. Corrupt files. This is similar to item one, virus' and malware, but it's not the result of a nefarious attack. Sometimes, a hardware failure will corrupt files or critical data structures (boot records, directories, FAT tables, MFT tables (the NTFS equivalent of directories and FAT tables), etc. It can happen, it shouldn't, but it can. The damage may be such that reinstalling windows is the best or only solution. THIS IS USUALLY CAUSED BY HARDWARE FAILURE (although it can be transient).
So those are the reasons why it really might make sense to reinstall windows. And, frankly, because of "register bloat", most users should expect that a "periodic" reinstallation of windows may be good or necessary. BUT, the "period", in the absence of malware infection or hardware failure, or people who compulsively install everything that they come across, should be 2-4 years (note, also ... this may be about the practical life of a typical comptuer due to the obsolescence that occurs as new comptuers become more and more capable).
A couple of comments: Reinstalling windows completely means reinstalling ALL of your other software (office, photoshop, multimedia tools, browsers, etc.). It also means you either backup your data first and restore it, or you lose it. If you have a lot of "stuff" on your computer, this will take hours and will probably take days. There are some strategies that you can use to minimize this:
1. KEEP YOU DATA IN A PARTITION OTHER THAN DRIVE C: This makes a lot of sense and is a good practice. But, unfortunately, it's surprisingly difficult. It means "moving" the "My Documents" folder from it's default location, and it means configuring ALL of your programs to place and look for their data in a partition on a drive other than C:. Software publishers (including Microsoft) do not make this easy, and even when it's not difficult, they don't tell you how to do it. And in some cases, the level of difficulty required to move a program's "default" data storage location reaches the point of manually editing the registry.
2. After you do a reinstallation, and have everything major more or less "right", MAKE AN IMAGE BACKUP. Then if you have to reinstall, the image backup may be hours or days faster than a piece by piece "bare metal" reinstallation of everything (restoring an image backup is usually a 20-40 minute task and it gets you back to where you were when you made the backup .... which should be shortly AFTER
you did a complete piece-by-piece reinstallation of Windows AND ALL OF YOUR OTHER SOFTWARE. The problem with this is that no only do most users not know either what it is or how to do it ... but it requires an image backup program that you will probably have to BUY. [Vista Ultimate has an image backup program in the OS, and in Windows 7 I believe it will be present in all versions].
3. CONSIDER USING "SYSTEM RESTORE". Very often, "system restore" can save your butt and avoid the need for the far more complex process of a full reinstallation of Windows. Another tool that most users don't know about and don't know how to use. But at least it's part of Windows, and it's free.