Oh help me, Techi Wan Kenobi, You're My Only Hope

I'm a bad fiancée. After spending a few months talking about doing something to cuvalwen's incredibly old computer (and not doing anything about it), I finally got round to transferring a lot of files off it (no room on the 20Gb hard drive) and defragging it. This should've made the thing a little faster at least, except there was no virus software on the computer beforehand and so I loaded up Avast.

Suffice to say the computer is now running *slower* and I'm definitely not flavour of the month.

[Yes, I did know this was an issue with Avast beforehand - and AVG, for that matter, but it was that or let the two trojans we found carry on, unabated...]

So, my wonderful *techie* friendslist, please to be helping. I'm in dire need of decent (preferably free, but not necessarily) virus software which doesn't make the machine run like molasses on a cold day. Is there such a thing?

Also - any tips or encouragement to get the damn thing running a little faster / better would be incredibly wonderful as well.

(To let you know what I'm dealing with, the vital statistics are: AMD processor, 600MHz, 386Mb RAM, 20Gb hard drive, Windows XP Home, Service Pack 3. Slower than a turtle on dope.)

You are all utterly wonderful!