Using iChat features...

Just about an hour ago I was chatting on iChat with one of my friends, and he has a MacBook. He was able to share his screen and invite me to audio/video chats. Yet, my iMac couldn't do any of it, not even the camera feature. I mean it has the iSight and microphone, and I don't know what screen sharing requires but if a MacBook has it, wouldn't an iMac? Is there any way I can fix this? I'm open to suggestions.

EDIT: iChat says I have no camera installed when I open the video preview window, however the green light comes on, and when I try to change my user pic on iChat it uses the camera just fine.