Unable to run Season 3 clients

I hope this is the right section to post my problem. I used to have this problem on my old PC and the problem came out of the blue without any logic. I did everything, updated drivers etc. but I was unable to fix this. I bought a new PC, which is way better than my old one (4 GB ram, dual 2.8 GHz processor, Nvidia GF 405 video card). I know that my video card is kinda old but I can run way heavier games than MU. Same thing goes for my old PC. As I already mentioned, this happens only on Season 3 clients. I can run all other seasons without any problems. Any help is greatly appreciated.

I wish icould check the video but on that mobile im not able . .if u can tell me what errors u get? Or what happens and whats ur OS?
Posted via Mobile Device

Well, the game simply runs extremely slowly, on Fraps I'm getting 1 FPS the whole time. I also can't see any letters (no server and channel names). I have Windows XP.

I guess no one has any clue as to why this is happening.