Because iTunes only supports certain formats. I don't use iTunes as a video player, but AFAIK it can play mov, mpeg4 or H.264 files. mov files aren't worth downloading via Gnutella, as they're mostly porn/spam: mov files
I've played mpeg4 files in iTunes before & they were ok. I just tried a H.264 video, but it was very sad & kept freezing. I don't have a .mov file to try.

I'd suggest that you forget about iTunes as a video player - there are far better ones out there. IMO, iTunes is a tool for burning, playing music & adding files to iPod. But then, I'm using iTunes 7 at the moment & I don't know anything about iTunes 8 or it's features.

If you want a movie player, check out VLC (free) or other players in this link: http://www.gnutellaforums.com/tips-t...p3-fixers.html
VLC has codecs installed to play most file types & is really reliable.

When you get your iPod, that will be able to play mpeg4 or H.264 files, so you're going to need a converter if you want to add videos in other formats to your iPod (eg .avi). I use 3GP converter, but there are other freeware ones around (Videora is a popular one). This link that I gave you earlier How to transfer music to my iPod (& iTunes) has info about adding video to iPod, converters etc