Personally, I am a fan of the "Computer Associates" Security Suite package. Anti-Virus, Anti-spyware, spam, and firewall protection. I think it is $49 per year, and it is unlimited updates. You can download the software from the Computer Associates Website, or find the boxed software at TigerDirect or other such store.
I have found that it is very effective and does not seem to interfere with running programs like some of the other 'name brands' I have seen or used.
Good luck. Hope this helped.