Any reason why you can't update to v20 SP1?
When I go to act downloads I see a bunch of ACT v20.0 and ACT v20.1 updates. I just tried to install Act! Pro v20.1 Update, act!_pro_v20_sp1x120.exe (574 MB). I get a message that says this app can't run on your PC.
Not sure if anything on this site applies to you, but it is worth a look.
Are there any HotFix text files in your Documents folder? Those are log files which might show the reason for the failure.
Finally had to contact support. They created a new user profile and were able to apply the update. Something in my profile was blocking the update.