It worked fine for months but suddenly...

It broke.
I've had FSX since October and it's been working fine, without a single glitch. Then today I fired it up and after it had loaded my flight and placed me on the runway it freezes. Freezes the game and causes all other applications to 'stick' and not respond. The machine doesn't totally lock up as I can still CTRL+ALT+DEL and try and end all the programs. It happens everytime it has finished loading and everytime I have to reboot at the CPU.
I have not installed any new aircraft/scenery since the last time it worked and havn't done anything to the settings. I can still access and work the intial menu and it works fine right up untill you actually start flying...
(Also, I've scanned my computer for viruses and the results came up clean)
Any ideas on what I could do to identify the
I've had FSX since October and it's been working fine, without a single glitch. Then today I fired it up and after it had loaded my flight and placed me on the runway it freezes. Freezes the game and causes all other applications to 'stick' and not respond. The machine doesn't totally lock up as I can still CTRL+ALT+DEL and try and end all the programs. It happens everytime it has finished loading and everytime I have to reboot at the CPU.
I have not installed any new aircraft/scenery since the last time it worked and havn't done anything to the settings. I can still access and work the intial menu and it works fine right up untill you actually start flying...
(Also, I've scanned my computer for viruses and the results came up clean)
Any ideas on what I could do to identify the