Suggest you view security programs in two categories:
1) Constantly running programs which protect your computer from infection.
2) Programs you run which will detect and remove various types of infections.
For the former, I recommend Spyware Guard, Spyware Blaster, Win Patrol, Microsoft AntiSpyware, Spybot Search and Destroy and a router (for your firewall) if you have broadband, or ZoneAlarm or Kerio* software firewalls, and AVG AntiVirus for its email scanner.
For the latter, I recommend Microsoft Antispyware, Spybot Search and Destroy, Ad-Aware SE Personal, A2, CWShredder, and AVG AntiVirus.
As you can see there is significant overlap between the two categories in that some programs accomplish both tasks, i.e. protecting and detection/removal. Please note that all the programs recommended are freeware. Also, I second DonNagual's recommendation regarding Norton and his emphasis on the regular backup of your programs and data.
In addition, you want to keep your system clean and tidy, and I recommend CCleaner for that task.
You can Google the name of the programs to find the download links.
Remember two things: 1) No one program will protect you from everything; you will need multiple programs to afford you adequate protection, and 2) the programs are rendered useless if you do not keep them updated constantly.
* If you choose the router for your firewall, but you still want to retain control of program associations (i.e. controlling which programs can be started by other programs), you can install Kerio which has a light profile and will not chew-up your resources.