My rule of thumb is that if it takes more than 2 hours to 'repair' a computer, I back up the data files (if possible) and reinstall the OS.
Your case could be a corrupt video driver; try rolling it back from device manager or reinstalling from the Dell driver CD. Check the boot log and event log -- may or may not be revealing. If the computer is still under warranty and you can boot in safe mode with networking you can call Dell and have them connect remotely for troubleshooting. You can uninstall the most recent Windows Update files. Lots of options.
My clients are happiest when I get the computer working again; they really don't care whether I figured out what the problem was, which can take hours if not days if not infinity
Back up, back up, back up. If you're ever faced with a reinstall you'll have a nice, clean, fast computer (for a while) and all your data.