VirtualDub (64-bit version)
If you want to tinker with some video, take a look at VirtualDub. This piece of Open Source software is well-suited for cleaning up, editing, and trimming video, and includes a host of tools for doing that. You'll be able to change the color depth, frame rate, and compression, as well as cropping video, and removing and replacing audio tracks without altering the video. It also does an excellent job of capturing video.
There are far more sophisticated capabilities as well--although VirtualDub better suited for cleaning up, trimming, and doing basic editing that for doing true video editing. Still, it's free and does the job. There's one drawback, though: It doesn't handle Windows Media files (.wmv).
Note: This software comes in 32-bit and 64-bit versions. This is the 64-bit version, which is for 64-bit PCs running a 64-bit OS. If your PC is running a different supported OS, please download the 32-bit version instead.