TKLK,
We need to get a bit more information to try and help with this issue.
What version of Vista are you using?
What version of Act are you trying to install and do you have any previous versions already installed?
Does this error only happen when attempting to install Act? If not what other programs have you noticed this error connected with?
Did you notice this error before ever attempting to install Act?
For a begining troubleshooting step, I would like to recommend that you clean boot your system and then attempt the install again.
Here is a link on how to perform a clean boot:
http://windowshelp.microsoft.com/Windows/en-US/Help/121ff6a6-181e-4e8f-83bb-c7b1f3c6d71a1033.mspx
Post back with answers and results.