Do you have any corresponding minidump files (unpXXXXX.mdmp) in the folder
C:\Documents and Settings\All Users\Application Data\Alwil Software\Avast5\log
?
I installed version 5 first time on my PC with windows 2000 SP4, but I never had installed version 4.8 before.
No problems anywhere and a super performance.
Have you installed ALL updates for windows 2000?
Is there any other AV-program or similar programm on your PC that perhaps is in trouble with avast?