And, finally, I have heard from people that UNINSTALLING SP3 which won't hurt your computer has helped.
Bert is right again.
It happened to us. We have Win XP pro Service pack 3 in our main PC where AC databases are. AC chart support staff Trish had to uninstall SP3 and get SP2 and then AC installation went w/o problems. She said that we should be able to upgrade to SP3 afterwords but I just left it like that for now.
