Thanks for the tips guys. Next time I'm round at my parents' house I'll try a multimeter on the PSU's ATX connector and molex connectors, I've no intention to open the thing up though, I need it to at least work until I can get a replacement.
That said, I had a similar issue to this a few years...
I put together an always-on server in my parents' house back in 2008, over the years I've added to it. Prior to two weeks ago the specs were:
AMD Opteron 1210
8GB (4x2GB DDR2 sticks)
ASUS M2N-LR/SATA motherboard
Three 7200RPM HDDs and a SATA DVD-RW drive
A 5-port PCI-X network adapter...