I think I've ranted about this many times before. AV software has caused me more problems than viruses in the past so I've stopped using it.
No doubt many people will now respond recommending an alternative product in good faith but it won't be much better.
The only time you need AV software is for scanning the odd program you've downloaded. It doesn't have to be running in the background, eating memory, CPU cycles and hogging Internet bandwidth to download updates.
If you can find a good free AV product which can be configured not to run in the background and only download updates when you tell it to, I'd be interested. I use ClamWin AV which I know doesn't have excellent detection rates but I like it because it doesn't do realtime scanning.
EDIT:
Have you looked into upgrading the RAM?
You could probably do that very cheaply. A few months ago, I upgraded my old PC rom 256MB to 512MB for £1.