WM,
http://www.lavasoftusa.com/software/adaware/
is a program you should have and use periodically, it will pick up a lot of the less-malicious spyware/sniffers you will download from the net. Just run it once every two weeks and you're GTG. I still use the free version and get great coverage.
Norton Antivirus 06 is the most important defence your computer has, so as long as that subscription is paid and you have it updating and scanning the computer bi-weekly, you should be fine (knock on wood).
JMO,
Solid