WIndows vs. Mac? Itunes 7 Problem?

Having this problem. Ipod formatted on Windows at home, was able to watch movies on my Mac at work. Downloaded Itunes 7, now can't watch anything, on Ipod or in my iTunes library on mac computer. Also won't let me add any videos I purchase to my ipod on the mac, can only watch and add on my windows. Anybody have any ideas?? Both my itunes and ipod are up to date.