If no application is using the RAM then it is going to the operating system somewhere. That would be file cache or a driver.
A video driver would be a likely culprit. Simply being on the latest version does not guarantee anything, because every bug was new some time. It could even be a combination with your hardware: the Radeon 4850 is fairly old at this point and AMD may not be testing the drivers carefully with that hardware.
File cache should show up in Task Manager. It's possible that the video playback is causing the file to be read into memory in preparation for playing the video.
It's possible that RAM use has nothing to do with your problem. Every Vista system I've seen reports RAM use to be very high. This is because Vista uses as much RAM as it can for program, DLL and file caching. It isn't normally a problem because this cached memory is easily discarded to make room when needed.