I am working with avast! on 2 PC's with Windows 2000.
You must install Service Pack 4 and Update Rollup 1 before installing avast!.
I use avast! V7 Free on one PC, the other has avast! V6 Free (but I will update as soon as possible).
And BOTH PC's are working without problems. So I recommend to install avast! V7.
Just for information: You can use avast! Free or Pro, but the "Internet Security" is incompatible with Win2k (due to a firewall driver).
Have a nice day!