Just wanted to point out a major distinction between viruses (and Antivirus software) and spyware (and antispyware software).
These two things are equally aggravating but very different problems. I'll leave you to google or wikipedia the difference but alot of programs will do one or the other, not both. Also there are many programs that "claim" to remove such garbage but actually put some in. So-called antispyware programs are notorious for this tactic. Always use something that is recognized and popular. YOU NEED TO HAVE BOTH TO BE RELATIVELY SAFE!
Suggestions for Antivirus: Symantec Corporate, Avast! (free), AVG (free), AntiVir (free)
Suggestions for AntiSpyware: Microsoft Defender, Spybot S&D, Ad-Aware.