Screen freezes suddenly


I have a problem with my screen (again).

The screen freezes during work or at the login(when I want to enter my password, it freezes), without any reason and I don’t know why and the only way is to kill the PC.

grafic card: ATI Radeon X300
resolution: 1280 x 1024 (SXGA)
Colors: 24 bit

3D Booster disabled.

no one who can help me?