FSX Freezes

Hi Guys

Lately I have been flying using VATSIM.
I have recetly instaled Windows 7 and have been flying using the PMDG 747-400.
The problem i have is, for example, today i was flying from melbourne to sydney, when i got 30nm to sydney, the computer freezes. The mouse still works, and i can still hear the engine and sometimes even the ATC but the actual flight freezes.
When i press ALt-CNTL-DEL, it says the FSX not responsing.
Can someone please tell me whats causing it and what can i do to fix it.

Thanks in advance