Well my OP still stands. If it is XP you'll be able to boot into the recovery partition and fix the computer from there. If it is Vista you'll have to get a Vista disc and reformat (as vista doesn't have a recovery option).
I did see that EricB posted a way to repair a Dell install of Vista - if I can find that i'll post the link. Either way you'll lose any/all information. So you'll want to backup the files (if you want to keep them) by using a Linux Live CD or taking the hard drive out and connecting it to another computer to pull the files off.
Edit: Here is the link to that thread i was talking about. I haven't tried this personally so i can't really give any advice in restoring Vista with Dell.
How to restore Dell vista computer to store bought conditions