Updating this thread again. Just tried to reformat the drive and did a fresh WinXP-install and the first thing I did was to install Catalyst 12.1(with the included SDK) and nothing else.
Then I started GUIminer and set it up just like before, and guess what? THE F***ING PROBLEM STILL REMAINS -_-.

What the hell could be the problem? Could one of the cores on the card be broken? Or could the problem lie with the pool rather than my computer? Or could it be some strange setting that I'm unaware of?
 
Might try installing Ubuntu instead and see if I can get it to work there.. If not I think I'm gonna sell the 5970 and get a good singlecore-card instead..