I coulldn't find instructions online so I booted from my 7 disc to figure it out.
1)boot from dvd
3)click "repair my computer"
4) select operating system/installation
5)the repair console will search for problems and attempt to fix them
6) if given a list of choices, do startup repair, or what ever makes the most sense.
If that fails, and you can find your driver cd/dvd that came with the computer, you can boot from the disc again and do a clean install. Things might look different, as Dell likes to play with a few settings before shipping out computers. Here is a link that explains how to do a clean install: Clean Install Windows 7