You might be having firewall problems, not the same version of AIM, etc. etc. You never know with Windows involved. So many things can/do go wrong.
Try using iChat (which I believe is AIM-compatible) and start a video chat through there. Make sure your system can handle the video chatting.
Please include more specs on your machine and possibly the PC you're chatting with.
I am trying to use the video chat, from a MAC to PC, through iSight with AIM. The issue that we have is the PC can see the video window from the MAC but the MAC can not see the video window from the PC.