How can I completely erase the hard drive?

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.

If You got installation media You can use ErAce to wipe hard drive. Download it from erace.it. Afterthat Your hard drive is blank and You can install OS from installation media

If it was me, I would replace the hard drive all together and reinstall Windows on a new hard drive and format the old drive. You will lose everything unless you already had it backed up though. However, if Dell makes has a recovery disc for your computer brand and model, you should order it and give it a try but you will still risk losing everything. Another method is to download a free anti virus program and a disk cleaner for junk files. I use Avast for anti virus and CCleaner (Spelled as it is shown) for junk files. Not sure if these good fixes but it is worth a shot, however, reformatting the HDD and reinstalling the OS are the best choices if you want a long term fix because I know that computer techs use this method all the time. Anyway, I hope this works out for you.

Remove the HHD and place it on a very strong electro magnet. It will erase everything. It might also make the drive unusable. You would have to reinstall Windows.

Your best bet is to install a fresh operating system instead of magnetizing you HHD. Just put a Windows or Linux disk in and do a fresh install formatting your drive.