I found an even better solution Jenny

I went on GoG and found a nice alternative. A Sierra "old-timer" chimed in with these 2 possibilities:
1- Either It could be a codec conflict issue, in which case it might have affected other movies in my gameplay, which it didn`t, thank goodness; He goes one to suggest temporarily renaming certain file extensions in system32 (shudder). Here's the link:
http://www.gog.com/en/forum/gabriel_knight_series/gk3_cutscene_crash2- The "bad" movies can be skipped altogether during gameplay and viewed using Rad Tools, which I did.
FYI, the short bit with Mose and Gabe in the hallway just after the faulty "morning after" bit seems to work just fine on my machine.
Does this earn me brownie points?

Mmmmmmm Brownies *drool*