If you are running Windows and need free antivirus software, I highly recommend AVG Free Edition. It has been much improved over the older free editions. I install it on all the computers I service that run Windows.
I think the URL is http://free.grisoft.com/ , but that is from memory.
Also, McAfee has a free online scanner you can use to locate viruses. It requires you to use Internet Explorer, but... whatever. It won't remove the viruses, but it will tell you what viruses you have, and you can Google to find ways to remove them. It's pretty handy.






