To reset the CPU speed you will need to enter the BIOS (F2 - del) on start up.
If you cannot delete files then a repair of the operating system may be required. REpair XP
in the run box, press enter and let system file checker find and replace missing / corrupt file in windows and explorer.
This will take awhile to complete and you may be asked for the CD to be in the drive if windows is not preinstalled on the hard drive.