For video's, get the DivX pack, it's free and it comes with a DivX converter, with no reduction in quality, so you can simply convert it to a DivX file. For music, I use iTunes, but that's only because I have an iPod. I find WMP faster, but I only listened to music on it. It also depends on how much memory your computer has, and how much the files are taking up. I recently did a major cleanup of my harddrive, and it made my computer run alot faster.
