If your computer is attached to the internet (duh), than you may have picked up a load of spyware agents. It's even possible someone has placed code into your computer that allows him to use your computer remotely for serious hacking purposes.
I would immediately get AVG Free antivirus, as I don't think the preinstalled Norton does more than slow down the computer. Also, Spybot and AdAware are other freewares that will go through your computer (once installed) and look for spyware.
Get these, do the scans, and remove anything they find. If your drive is still busy, get What's Running (freeware), which will report on all the apps running in the box.
The bad news is that there are now rootkits (so called) that install themselves and run the normal processes at a virtual level, so the rootkit is invisible. If the HD stays busy and Dell says it shouldn't be, you may have something like a rootkit. Scans may report stuff in the registry that can't be removed, which is a good sign of a rootkit. If you are familiar with the registry, a manual scan with REGEDIT may let you see something odd. If it's locked and can't be removed, you're in trouble.
The cure is to back up everything and blow the drive. Don't just resotre it - format it first and use the system restore discks afterwards. You might want to get more advice before you jump into this.