Okay, I have set up a very quick guide on how to show all the threads. It is even better than the one BendiK posted.
Just follow these two images: (You have to copy+paste them into your browser, because they are 1280x1024 pixels big, so they ruin the design of the forum. )
Click at User CP at the top left of the screen.
Then click Edit Options:
Then scroll down to Default Thread Age Cut Off and set it to Show All Threads (Not Use Forum Default as it shows on the picture below):
Then scroll to the bottom and click Save Changes, and it should be fixed at all the forums.