HEUREKAAAA!!!!
I fixed my problem, but I dunno how.

IS that redicoulus? well, yeah...
Anyway, I tried the last thing that can be updated. The so called BIOS(As I suspected), I spent spent lond time identifiing my BIOS type, but ifound it and updated. The decription said something about it fixes errors that causes some Nvidia cards to malfunction. After the flashing I decided to set the BIOS to "Optimized defaults". I dunno wich one helped, but belive it or not, HL2 and the Sims2 runs like never before!!!! No freezing, no stuttering, beautyful(well...as can be

) graphics, no lock-ups, no errors. Just a few "crashing to desktop with no error messages" stuff, but not too often(quicksave rulez

).
The BIOS was the bad guy, as I suspected.
Thanks anyway