The old forums have a thread for installation tips for vista here
http://forums.biowar...620091&forum=10
Win7 suggestions were mostly the same, though there is a separate thread here
http://forums.biowar...700768&forum=10
the summary of it is with default Vista settings, the security settings restrict what the user can do in terms of installing new programs, so that's why people recommend disabling the UAC or setting the file to run as administrator (or both).
There are also different security settings on anything under the default "program files" folders, which is where BG2 and TOB want to install by default. Changing it to a "games" folder right off the root of the hard drive and NOT in the "program files" folders will eliminate that issue as well.
Both the threads above have much more specific info there, give them a read.