Can't play/download purchased or rented movies?

Hello all. I've had a problem for quite some time that I never solved. For some reason my Mac and iTunes has never worked right with movies.

If I were to rent one for example, the download bar changes to the familiar rotating candy-cane effect almost immediately, and then I a told to click to being watching the movie already.

Of course this doesn't work, and waiting for a lifetime has no effect.

I am curious if anyone were to know the exact problem here, I have tried everything and this one seems to beat me. A search on the net does not seem to bring this particular case up. I have already changed my ISP and that was no help. Everything is up to date.

Thanks Just, I did try that as well after other things failed, and I had the same problem. This just totally stumps me. If I use another computer on the network then there are no problems. After moving to another location the issue still exists.