Login screen freezes

My screen freezes when the login screen is loaded, but the computer does not freeze, because I can type in my password (I can't see the dots though) and hit ENTER, then it logs in, but the screen turns into one big mess after that. If I click the power button on my computer it hibernates. When turn the computer on after a hibernate, everything works as it supposed to do. The screen only freezes after a shutdown. The computer seems to be having a hard time running heavy graphics lately.

I made a clean Windows 7 install for about 6 months ago and I haven't used the computer much since then.

Here is some information about my computer:
Dell Studio XPS 1340
Windows 7 Professional 64-bit (6.1.7601 Service Pack 1 Build 7601)
Intel Core 2 Duo CPU P8700 @ 2.53GHz, 2534 Mhz, 2 Core, 2 Logical Processors
NVIDIA GeForce 9400M G (Driver version:

What can I do to diagnose the problem so I can fix it?

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