Personally i use Microsoft Anti-Spyware (it is now included in Windows Defender (Beta 2)) availiable at www.microsoft.com
and there is a link on the main page.
I personally also use Ad-aware SE www.download.com
and look for Ad-aware SE.
As for Anti-Virus i use AVG Free which is great!
You will also need a firewall which i suggest Zone Alarm Free www.zonelabs.com