Assuming your hard drive is toast (unlikely from a simple virus), you should be able to replace it with any standard 2.5" PATA hard drive. This is probably your best bet:
It's cheap (about $65 shipped), it's fast (5400RPM, 8MB cache, your old hard drive was probably 4200RPM with 2MB cache or less), it has plenty of space (80GB) and it has a 5-year warranty (best in the industry). I installed one of these in my parents laptop (going from a 40GB 4200RPM), and this (along with +1GB RAM) really gave it some new life.
You will need to salvage your drive caddy (it's attached to your existing drive via a few screws, it's like a metal adapter or cage that surrounds the drive, makes it easy to mount and unmount) and possibly the PATA adapter, if Dell uses those (it's a little adapter that plugs into the drive). Then you just connect them to your new drive, plug her in, and reinstall the operating system of your choice.