Maybe your PC won't boot. Or it could be infected with some kind of persistent malware. Perhaps Windows puts up a Blue Screen of Death every time you type the word 'aardvark'.
No matter what kind of trouble your computer is having, someone will probably tell you to reformat your hard drive, reinstall Windows and your applications, and then restore your documents, photos, and music from a backup.
That isn't always good advice - according to a 2004 Salon.com article, incompetent tech support people use it to get customers off the phone. But if nothing else fixes a problem that's impossible to live with, starting over from scratch may be the best solution.
Don't do it unless you've tried everything else, however. If your PC refuses to boot, see 'Diagnose and Repair an Unbootable XP or Vista PC' and 'The six best free programs to save your failing PC' for better options. If Windows boots and then horribly misbehaves, check for malware, remove unneeded autoloaders, and update drivers. You might even consider cleaning the registry.
If after all that you determine that you must wipe out and reinstall Windows, follow the steps over the next few pages to make the process as safe and painless as possible.