Why are my videos not synching to my iPhone

I've got an iPhone 3GS and the latest iTunes on a Windows 7 PC. The problem I'm having is synching videos from the computer to my iPhone - even when checked they do not sync. I made sure that the videos were MP4 (as well as tried other formats) and that there is enough space on the device, but still nothing. I'd really appriciate help with this, it's really annoying.