You're gonna want to clear your hard drive.
First, get a USB flash drive (jump drive) or other storage device (burnable CD, external HDD). Copy all of the important files you still NEED to that device. This includes important documents. You will also want to copy any items you may have from iTunes, its ridiculously challenging to sync an iPod if the items on your iPod aren't in your iTunes library. Also, if you have an iPod synced to that computer, sync it one more time to make sure all of your iTunes items are backed up.
Now, you should find the restore tool on your computer. Almost all computers have a built in recovery partition or came with recovery discs. If you don't have either of these, check your computer manufacturer's website. You should be able to order recovery discs, some cost money, some are free.
Run your recovery program, following any instructions either given on screen or by the manufacturer. Once done, your computer will act as if it is brand new. Move your important data back onto it from your jump drive (or other storage device) and you should be back up and running!
Oh, one last thing. If you do think this is caused by a virus, restoring the computer will remove it as well.