Fixed the problem in skyrim by reinstalling it (Doesn't work for Oblivion). However, I noticed one thing when I had the problem in skyrim; When skyrim's fullscreen works properly, you have to alt-tab the game 2 times to get back to game. However, on the first alt-tab, you get a black screen with the exact same borders as I have in Oblivion. I need to alt-tab only 1 time to get back to oblivion, and needed to do that only 1 time when I had the problem in skyrim, too.