The telephone socket downstairs has gone on the blink, and I can't find the fault. Also, why do some high-def mp4 videos play fine on my mum's laptop, but have the audio 3200ms ahead of the video on my desktop?
Graphics card/chip in the desktop isn't rendering fast enough. You can try updating the driver or updating the card if it isn't integrated into the motherboard.
The laptop has integrated whatever-graphics-chip with drivers that are who knows how old, while my desktop has a Geforce GTX 660 with the latest drivers. I've had this general kind of problem since before I installed the GTX 660 too, back when my desktop just had integrated graphics, so I think I can rule out the graphics card as being the culprit.