avg and windows 2000 sp4

i have a small network in which most machines run windows 2000 SP4.
none of them have the rollup pack. in terms of os these machines are
octuplets. i succesfully installed free avg 8.0 on a few of them and
everything went well. however, when i ran the .exe on others i was
informed that the os was incompatible. which has me baffled. i double
checked the spec by running and printing out belarc profiles and it
confirmed that my machines are identical. anyone have any clue on how i
can fix this? should i download a different antivirus?(it has to be free
because this is a non-profit with a tiny budget).