There are almost too many possibilities to make reasonable guesses without hands-on examination of the machine.

1. The symptoms you experienced before reinstalling Windows may, or may not, give some indication of whether there was malware installed that you didn't detect and/or whether you have components that are marginal or failed/failing.

2. The complete method you used to reinstall Windows might give clues as to whether your reinstallation was "clean" enough to avoid duplication of files, cross-registrations, or other installation problems that could cause the symptoms described.

3. A reinstallation may sometimes be seen as a "new machine" and registering with Microsoft may be required to get/keep full function. There is a "grace period" of some few boots before some functions are automatically disabled for unregistered use; but if the registration is invalid lots of things will "go away." These is something of a problem with WinXP-Home, and a possible but lesser likelihood with WinXP-Pro.

4. If you installed from an older installation CD that does not include SP-1 and SP-2 you may have already picked up malware that exploits one of the holes that is patched by these two updates. Later security updates are necessary but usually attacks are not too frequent to get them on a more leisurely schedule.

5. If your Anti-Virus was not current the first time you logged onto the internet after you reinstalled Windows, or if your AV is not running properly, you may have been infected with something that interferes with your machine.

That's the short off-the-cuff list.

If you can pick one or two possibilities and provide more information it may (or may not) be possible to suggest remedies. For some, about the only thing that can be suggested is to get local help.