Windows 7: Win7 intermittent Freeze after login

Hi
Got a problem with a Windows 7 64 bit home build that I made for my cousin which I'm really hoping somebody can help me with.

Kit is :-
asus p7p55d mobo
i5 750 cpu
4GB corsair 1333 DDR3 RAM
500W coolermaster psu
sapphire 4670 graphics
3x WDC SATA HDs set up as AHCI
win 7 64 bit.

Before installing any software I ran the latest memtest for a couple of hours and all was fine.
All that had been installed on the machine is Windows 7 64 bit and office 2007 professional (plus all updates for both).

All the software has been updated to the latest standard as has the BIOS and motherboard drivers.

It appears to boot to the login screen with no problems all the time, then one of two things will happen :-

a) It will login fine and run all day with no issues (apart from a few occasional excel and outlook issues which may be another issue).

b) It logs in and the desktop background and icons appear but then it just freezes.
there's no cursor and no response to any key presses. Theres no blue screens or evidence of corrupted graphics.
Only a hard reset will get it back.
After re-booting, there's nothing I can see in the event logs apart from the fact that it didn't shut down properly.

When it does start up fine (about 75% of the time) I have run 3dmark2007, pcmark2006 and the intel burn-in test prog and it copes with them no problems at all.
Temperatures vary from 19deg at idle to 58deg (using latest coretemp) when the burn-in test was hammering it so they look fine to me.

I believe the hardware must be pretty solid otherwise i doubt it would cope with the various test progs I've flung at it and I can't see why HW would cause it to freeze at exactly the same point in the boot every time.

Any ideas please guys?

cheers
Mark

Hi Mark

Does it boot correctly in safe mode (F8)? if so may be as simple as video driver.

Let us know

Ken

I won't get another chance to look at it until the weekend.

But the whole point of me running all the 3dmark & pcmark stuff was to give the system a good work out.
I would have thought any video issues would have shown themselves up then and during normal running.....it's solid when it is up and running and gets left on and used for long periods of time.
It ONLY freezes (sometimes) just after the desktop has loaded and icons appeared...never anytime else.

cheers
mwa