btw, although you can put your video and music on a USB Key or external hard drive, the same does not apply to games.
When you install a game, not only does it copy the files to your PCs hard disk, but it probably also updates some files within Windows (like the registry file for example).
If you had a new Windows install, and just copied the games files accross to the hard disk I would guess a lot of the games would not work.
(I am not a games expert but the above is certainly true for other types of Windows programs so I assume it is also true for games).