You should Boot into Safe mode & run various Antispy
/ Antivirus programs .. such as Spybot SD, Adaware SE Personal,
Spyware Blaster, a good Antivirus Program such as AVG Free &
also a good Trojan hunter such as Ewido ... it is possible your computer
picked up something that is causing all this.
Remember to disable System Restore & empty the temp folders
before you start the scan ... if that does not work, next thing to do is when you reboot, enter BIOS setup and set it boot from the CD first, Hard Drive second ... Next, reboot with the windows xp cd in the drive
Select the Repair option .. try to have windows do a repair install
See if that helps you
Hope it helps