Original post by: Dan ,
If you your system came with a recovery CD/DVD disk you should be able to delete and reformat the drive with a fresh copy of OS & the Apps that came with your system. If your system didn't come with a recovery CD it may have a recovery partition or you may need to order the disk from Dell. I would start there. If you can't go that route you could give one of the antivirus applications a try. You also may want to setup a bootable USB drive and then booting up under it defrag your HD. While viruses are nasty things you shouldn't need to low level format the drive or need to replace it.