I have a new system that's having a few problems... sometimes (seems to be when the PC is cold, i.e. has been switched off for a while, though that could be my imagination) I get a blank blue screen when I boot up.

The system boots normally and auto-logs-in. The desktop loads and I'm even able to launch applications, but then everything disappears and the screen goes to the default windows desktop blue colour (not the desktop image, just a plain blue with no mouse cursor). At this point the machine completely locks up - I'm unable to even toggle Num Lock and have to hold in the power button for 5 seconds to kill it.

Interestingly if I manage to launch some applications before it goes blank, they will usually crash... sometimes explorer.exe will crash too.

When I reboot, the system is fine and stable. I've installed the latest graphics drivers and run memtest86+ for 6 passes (and counting) with no errors.

The system specs are:
CPU: Intel I7 2.66 @ 3.4GHz
RAM: 6GB (3 * 2GB DDR3)
HDD: 128GB Crucial M225 SSD
Motherboard: Gigabyte EX58-UD3R
Gfx: ATI Radeon Sapphire 5870 1GB

Note: There are a few similar questions but I haven't found one that matches my symptoms