Norton is probably the most popular Anti Virus software, but later versions have been reported to slow down PCs quite a bit. I have recently switched to
F-Secure Internet Security 2006, which is an all-in-one Anti Virus / Firewall / Spyware and Spam protector. I have found it to be excellent, but unfortunately it's not free. You can, however, download a fully functioning 1 month free trial version from
here, which should be able to clean up your PC, then you can decide whether to purchase the full licence, or look for something else. If you do decide to buy it, you can get it cheaper from
Dabs, than from the F-Secure site. Whatever you choose, be it paid for or free, you do need to have anti virus protection and a firewall installed on your PC, and kept updated. The paid for versions do automatically update, so once set up, you can usually forget about them, but some of the free programs require manual updating.