Two helpdesk techs here, and no one recommended the easiest and first thing to try. He's on Windows XP, he's got System Restore.
START, ALL PROGRAMS, ACCESSORIES, SYSTEM TOOLS, SYSTEM RESTORE.
Now...click on "restore my computer to an earlier time". A calendar will pop up.
pick a restore point a week ago or something, when you know your computer was working well. You will not lose any documents, emails, downloads or files....you will lose any new programs or system settings that have made their way onto your computer since then....and you'll have to reinstall or reset those.
Give that a try before reformatting or something and let us know if it works.