I've just picked a second hand hp pavilion ze5400 lappy, running XP home v.2002 Service pk3, 2.4Gb P4 CPU, 1Gb RAM & 40Gb HDD. It has on it already, Avast Pro 4.8. I have no previous experience of Avast, but have an idea that it's somehow heavy on resources esp with all the 'On Access Protection' ..... I normally use AVG free which seems alot lighter. Is this the case or just my imagination??
Unless you like paying for what you can get for nothing? I've been using it for years and have no complaints. While you're about it you might want to add Spyware Terminator also free. In fact you won't have a dilemma with that, because they don't do a paid for version...click here
The difference between Avast Home and Pro in usage is minimal. It runs in the background, checking inoming's and outgoing's. I don't believe it to be any more or less resource hungry than any other AV. It does it's job and most of the time I don't know it's there.
It's not like you're thinking about buying it. As you say, you already have it. Try it and see what you think. Open 'Task Manager' and check to see what it's using.
I'm running Avast Home on my laptop and MS Essentials on my desktops. The machines are much happier this way around. MSE slows down the laptop noticibly whereas Avast does not, yet on the desktops the opposite is true. I think you need to do a 'suck it and see'.