Could be as benign as XP "indexing" folders, or could be as bad as a malicious program collecting all your passwords and personal data to be downloaded via Trojan to some online dirtbag the next time you log onto the net.
As mentioned- find out what processes are running in the background, run Spybot & AdAware for spyware detection, run an antivirus scan, etc. & hopefully you can figure out what your system is doing. Good luck...