I suggest at least 2 - I used AVG {free version} and ended up with 4 viruses- auto updates and all.
Bit Defender - {demo} download was the one that finally found the problems.

any of the brand name AV should be good- Pcillin, bit defender,mcafee, norton, kaspersky - you can usually go to the websites and dnload a demo- to try and see if you like how it is for you.

Spybot has been a good spyware program for me. I just do a manual scan once a week or so with it- it's not running all the time.

I'm using mcafee suite now and it does use a lot of resources so I uninstalled it and then reinstalled it - only loading the parts of it I really needed.
much better now.

netscape 8.1 {free browser} has some built in security features- you can turn them on/off and it will run a scan when it installs too.
it found a trojan.

marijo-
AVG - I had the same thing happen I uninstalled and reinstalled it too- i think a file got lost or something- but eventually it missed the viruses that ended up crashing my hardrive - or when i deleted some of the "bad "files I might got rid of something important - cause it crashed a week later.