First thing I would check for is a failed hard drive. You can test it in the bios(F2 for Dells).
If that passes, you can try to repair Windows if you have a system repair disk. If you don't have one, you can make one on another computer that has the same version of Windows.
If this doesn't do it, reinstall Windows.
Hopefully you have the recovery disks, as well as having your data backed up.