I've been looking for this for a long time. On Windows Vista, there is a folder called "WinSxS", which stands for "Windows Side by Side". It's a part of the move to a "component" based architecture, where the WinSxS folder contains multiple versions of a component such as Windows Media Player or IIS, and the appropriate versions are hard-linked into the system. Unfortunately, Windows is stupid (surprise!), and provides no easy way to remove old versions.