I know that it you go to the edit details you can see all of the clips currently used in your project. Is there any way to find out if any avi file is used in ANY project at all. The idea being that when I find space getting too tight I can see what avi's I could get rid of.
Nope, not really, because at any point, it's just a file on a drive. Drive doesn't know that the file is being used in Vegas, Premiere, or anything else, unfortunately.
You can eliminate files that aren't associated with prerenders, but there isn't a way of knowing if a file is currently assigned to any given project without opening that specific project.
This is where using good media management at capture/editing comes into play.