Do you have a Windows Video (.wmv) and need it in Quicktime (.mov)? Or vice-versa? I found a FREE video converter program called Prism. And I mean really free. Often when you download something like this, it lets you download, but then when you go to use it for real, it makes you pay for it. So, if you need one, check Prism out. It does a variety of conversions.
If you know of other free conversion programs, offer them up in the comments. I've only tried a few; I'm sure there are other great ones. (for free)