Had this a few weeks ago. The advice I got (which worked) was to boot in safe mode, then delete the user (there was only one). On restart it worked fine. Other alternative is F8 on start (I think for a Dell) then try a repair of Windows.
Hmm - odd! You should be able to access the page where you have the option to repair or return to factory settings. If you can't, and you don't have a disc, you'll have to scrounge one from someone. No problem as long as the key is still on the case somewhere.