I found out that the problem was coming from my hardware. However, my graphic card (ATI Radeon 4850 HD) has the latest drivers, and so does most of my hardware. I once had the same problem with the first video of TES IV: Oblivion (are every Bethesda games like that?), and I simply had to download the hotfix for my graphic card. Today, I tried Oblivion, and, unfortunately, the problem was back. But now, I got no idea how to get rid of it, and this is making me sick.
I'll soon upload a picture to show you the whole thing. Until then, I beg you, try to find a solution. If you need any information about my hardware, just ask.
Thank you,
Jasmeru
Edit : Here's a little pic. Notice that the upper left corner is a bit further than the rest of the video.
http://img193.imageshack.us/img193/4855/fallout3picbug.png