Windows 7: Assassin's Creed freezes

I just installed Assassin's Creed 1 and after playing fore approximately 20 minutes the picture froze and sound started flashing and repeating one second sound again and again. Alt+TAB didn't work, window button didn't work, I waited and at the end I had to turn off my note book the hard way - holding down power button.
Does anyone have idea what might be causing this problem and how can I fix it?

Hi synezid, Welcome to SF.

Is this only happening in Assassin's creed or in any other game as well? When the game froze, did you get any messages in your bottom right side like "Display drivers has stopped responding...." ?

Keep us posted.

Thank you very much.
This is the only game that this happens to, and I don't get any message. The picture doesn't change from the moment when the freezing happens, and I'm unable to do anything with my computer.

Have you updated your audio and video drivers? Being a Ubisoft game, I would suggest downloading the patches and then installing them (if there are any), for the most part, doing so automatically creates more problems than it solves. Also, if you are only playing the Campaign I would suggest not bothering with the patches as again, they break more things than they fix... my personal opinion of course.

Another thing I am going to suggest is going to the AC Ubisoft forum to see if others have the same issue and perhaps a solution for it.