Looks like everyone before me covered most the bases already.
Booting into safe-mode and running System Restore to try and get back to a working configuration would be the first thing I would try.
You might also try booting from the Windows CD to try repairing Windows. Unfortunately, it might not help and there is a small chance you might loose some of your data, but it is another option. If you have data you don't want to lose I would highly recommend connecting the hard drive up as a secondary to a computer with good virus protection to back up your data.
If there is nothing on the hard drive you have to save you might consider just using the Image Restore (aka PC Restore) option to reload the Dell factory image. To do this, just press CTRL+F11 just after powering on the computer and it will wipe all the data from your hard drive and reload the factory image.
Dell Customer Advocate