If it's as infected as you say, it may be a lost cause. First question is, can it boot from a USB key? There are some linux distributions you can slot onto those and boot from them, leaving your cd/dvd burner free to use as a back up mechanism. This would allow you to mount the HDD and save whatever you wanted to off of it, like bookmarks, documents, etc. Once you've done that and burned them to optical media, you can proceed to blow away the HDD and start from scratch.