Have you been afflicted with malware in the past?
One thing you can do is grab your Windows install disc, or make a new one of the same exact Windows XP installation you have - moderator note: this is not illegal at
all, since all you are really paying for is the right to use the operating system (serial key). Microsoft provides ISOs of their operating systems. Every link to Microsoft OS's
here is microsoft.com.
You can use the CD to initiate a repair, which will replace corrupt files with their originals. If you have not modified anything within your operating system (themes, explorer, etc... I fuxwitdat all the time) then you should not notice much change, aside from the fact that everything will work.
If that fails somehow, reinstall windows. That is the fix-all solution. If you want some personal help, just PM me or message me on skype.