"This only seems to happen if the PC has been sitting switched off for more than an hour or so"
This is the classic symptom of BAD CAPS. Google BAD CAPS and inspect motherboard, video card, power supply and ALL CARDS for any dome tops, blown bottoms, tilters, leakers and all the other signs of failure.
My desktop is giving me some problems on initial start up and having searched the internet for days for a solution I find myself here asking for your humble advice.
When I power up, the PC the screen is badly distorted and pixelated and then proceeds to crash when the windows logo is on screen (barely visible). After switching the PC off and on at the mains it starts up fine and there are no further problems at all (this sometimes takes 1 or 2 attempts).
I have removed, dusted and reinserted the graphics card and all drivers are up to date but still the problem occurs. This only seems to happen if the PC has been sitting switched off for more than an hour or so. I have had the PC for around a year and this problem started occurring just a few weeks ago.
System info is as follows
Operating System: Windows 7
Processor: 2.90 gigahertz AMD Athlon 64 X2 Dual Core
Graphics Card: NVIDIA GeForce 8600 GT
Main Circuit Board: Gigabyte Technology Co., Ltd. GA-MA69VM-S2
If anyone has any ideas it would be very much appreciated.