A range of things could cause this.
Try booting in Safe Mode. to do this press F8 just after power on. You should see two menus. The first asking what mode - you select Safe Mode. Then it may ask you which system - usually only one: Microsoft Windows XP Home... or Professional.
Once in Safe Mode make a new user: Control Panel | Users.
Then reboot. You may be able to log in with this new, clean profile.
Alternatively if you are in the position to re-install then try a complete scrub then install. That is - format the hard disk during the installation. All files will be deleted. This includes My Docs, saved passwords, MS Office, etc....
I guess there is a corruption in the registry. Formatting the disk puts a new unadulterated registry on.
I have a chum that does this every year just to keep his PC sprightly!
Eoin, November 2007
