Don't use Avast in light of what you just stated pertinent to your MSE experience.
AVIRA FREE EDITION is the one for you, so long as you do not mind and occasional false positive on agressive mode. I have been using Avira for over 2 years. I love it. It does not drag my system down whatsoever. I am the type of person that will not tolerate even a 1 or 2 second hang. Window hangs, sticking virtual buttons and general slow downs drive me nuts, so you can trust me when I say Avira WILL NOT slow you down. Besides that, it has been a solid AV program.
You will want to disable the splash screen that will appear at start up. It is a simple registry tweak:
"HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run"
the "value" for the avira "avgnt" "regkey", there, would be:
"C:\Program Files\Avira\AntiVir PersonalEdition Premium\avgnt.exe" /min /nosplash"
note that there is a space between "/min" and "/nosplash"
(Splash screen removal complements of Avira support) http://forum.avira.com/wbb/index.php?page=Thread&threadID=89419
I use 2002 XP sp3, so this method should work for you as well.