Well, not for the first time my wireless card is misbehaving. Strangely though, this time it hasn't automatically recovered, like it's done before.
I did some experimenting, and I've narrowed the issue down to my PC itself. The router's fine, reception is fine (a laptop can connect from where my PC is), but my PC's card isn't.
When I first boot the PC, the LINK light does light up, but goes off before I can log into XP. The LINK light remains off, and the Tx/Rx light flashes 7 times every so often (unless I say 'Search for wireless networks', when it flashes more manically).

Anyone got any ideas? Like I said, before it's fixed itself, but not this time.

It's an Edimax card I think, with a RealTek RTL8180 chipset.

Try taking out the card and then put it in another PCI slot, it may be that the PCI slot you have it placed in isn't properly connected to the motherboard, or that the card is slightly loose.
If that doesn't work you could try updating the drivers for the card.

Tried the PCI thing - nope.
Tried driver thing - nope.
Tried a different card - nope.
Rebooted the router - suddenly starts working.

I tell you, that's the last time I get anything Edimax. I've had far too much trouble with the router for it to be worth it.