This would be a good place to start http://www.avast.com/eng/system-requirements-avast-home.html, avast system requirements are quite modest in comparison with other AVs.
A 486??!! 
I've installed Avast on many a system and I wouldn't advise anything less than a Pentium II with 64 MB's of memory, preferably 128. Avast 4.6 even impacted my Pentium III system with 384 MB's of memory! The only system I've seen absolutely no impact is my Best Friend's (Pentium 4, 1GB RAM) laptop.
Who wrote these minimum specs anyway?
That is a minimum spec, and avast should run on that, not to mention it will run on the older OSes also, unlike many of the other AVs. Windows also has a minimum specification, which many would consider unworkable, but it works albeit slow.
If you have lots of software installed and running on boot (you are going to run short on system resources), it makes a mockery of any minimum spec and you have to look at the collective minimum specs of your system to achieve what would be a realistic working specification. avast, can't determin what else you run on your system, it can only state a minimum for avast.
As for who sets the minimum specs, I guess someone at Alwil and I woud also assume that these specs are tested.
I don't have a hugely powerful system and avast makes virtually no impact on my system, but then it is well above the minimum spec for an XP OS.
