I suggest setting the screen saver and monitor to something reasonable, like 30 minutes or so. If you leave the PC on all the time, set the hard drives to spin down after an hour or so.
I can't stand AVG anymore. I don't think it is very reliable, mostly due to all the false-positives it's given me over the past year or so. I have a 2.8 Pentium4 and a gig of RAM and Norton AV 2005 made my system crawl when realtime detection was turned on.
I'm using McAfee 2005 AV and firewall. So far, I'm really digging them. Comcast offers them free of charge to subscribers. Unlimited updates as well, except for the Spam Killer software which is only offered as a 30-day trial.