Ok, I just convinced my mother to let me open up the family PC to clean out some of the dust, as it's not been done in ages.
I took the case off, took some dust off the case.
I then took off the CPU heatsink, and cleaned that, then screwed it back on, and closed the machine up again.
Now it wont work!
It powers up, but I just get a blank screen. It won't load the OS, and even the option to go into Safe Mode or my BIOS doesn't appear.
Mam has said that I have to pay for the repairs myself, so I'd appreciate it if anyone could think of what I could have done wrong.