Ok, I've had this happen to one of my computers at work. It was cause by spyware being on the computer.
First run a couple of scans for spyware. Spybot, Adaware, SpySweeper whatever. Then go into your add/remove programs and look for obvious spyware (like ad deliverer, toolbar etc..)
Then to fix this problem, you need your Windows CD. Put it in your CD drive, the goto start -> run -> cmd
type the following
expand CD-ROM Drive Letter:\i386\config.nt_ c:\windows\system32\config.nt
expand CD-ROM Drive Letter:\i386\autoexec.nt_ c:\windows\system32\autoexec.nt
expand CD-ROM Drive Letter:\i386\command.co_ c:\windows\system32\command.com
Replace CD-ROM Drive Letter with the actual drive letter (it's probably D)
For more information about this, here's the microsoft article regarding the issue.
Go to Start -> command