It seems not an easy task to find a working converter for Vista! Did a quick Google, and of course about 300 programs popped up on some suspicious sites.
But I really need to convert a few protected iTunes videos. I tested a couple of sites and detected MelodyCan converter. Their free trial is quite impressive and allows converting video and audio formats both protected and standard ones. The main requirement is the ability to play the original file in Windows Media Player (for WMV) or QuickTime Player (for M4V).