Well this is embarrassing. A new computer (from Overclockers) and it goes kerplut after a month. Oh well, here are the specs (I can't really remember the drivers but they should all really be up-to-date because I update them fairly regularly):
Motherboard: Gigabyte EX58-UD3R Intel X58 (Socket 1366)
CPU: Intel Core i7 920 2.66Ghz D0 (it was originally overclocked to 3.3GHz but it couldn't handle that quickly so I switched it to 3.0GHz, and in an attempt to solve this problem I reset the BIOS by taking out the CMOS battery for a while so it should be back to stock speed)
Memory: OCZ Gold 6GB (3x2GB) DDR3 PC3-12800C8 (1600MHz)
PSU: Corsair TX 650W ATX2.2
Graphics: ATI Radeon 4890 1024MB
Sound Card: 7.1 High Definition OnBoard
Hard Drive: Seagate Barracuda 7200.12 1TB SATA-II
Operating system: Windows Vista x64 SP2
So here's the problem, one day it just didn't start up properly. Hadn't installed any programs or messed with drivers or anything like that for a while either. Inside the PC I can see the all the fans going, so I didn't think there was a problem with power. The monitor comes up blank, but it recognises that something is attached because otherwise a little message pops up on the screen saying nothing is connected. So I checked the power and monitor (with cables) with another PC and they work fine. I replaced the Radeon 4890 in the PC with an old Radeon X800XT that I know works, and checked all the PCI-E slots, but still no change (so it probably isn't the graphics card). So then I transferred the hard drive to my old PC (which I'm using now) and it booted up fine with everything intact. Got in contact with OcUK support and they told me to check the RAM, so I lastly checked each of the RAM sticks in the new PC individually using different slots, but still nothing happened. Waiting for them to get back to me again, but in the meantime any suggestions (I'm hoping it's nothing ridiculously stupid I forgot)?