has been promoted for years, but it is simply without foundation as it is generally presented. There are only a couple of issues that can't be rectified with a repair/reinstall, and these occur very rarely, and are usually the result of ill-advised tampering with certain system files and/or the registry. Click on "Registry Cleaners" for more information about the registry.
While it is true that Windows and other applications create lots of temporary files and log files, these can be dealt with rather simply, and certainly do not require a fresh reformat/reinstall. Click on "Disk Cleanup" for more information about cleaning out unwanted and unneeded files.
Poor performance, slow response, lagging, long boot times and such are most often the result of the user, not the operating system. Multiple toolbars, interactive desktop weather, so-called "quick launch" files for certain applications loading at startup and other such unnecessary clutter is what builds up over time, and much of these items result from user choices, not from the age of the O/S installation. These can be trimmed down and/or eliminated by the user with a few simple procedures that can restore system performance.
Many booting problems can be corrected using the Recovery Console. More serious issues may require a repair/reinstall, and there are many guides available on the internet. My favorite step-by-step guide, complete with screenshots, is here.
But the reformat/reinstall myth is just that. With some education, a little help here and there, and a common sense approach to routine PC maintenance, XP can be resurrected without resorting to the reformat/reinstall dance. And I say "dance", because there will be many partners; all pertinent XP Updates will have to be redownloaded and reinstalled, all drivers will have to be reinstalled, all applications will have to be reinstalled, all data backups (you do make backups, don't you?) will have to be restored. It's a long and tiring (and quite unnecessary) dance.
It's a dance I haven't done since Windows 95 OSR2, and you don't necessarily have to do it either.