Well, I went into some officially-whatevered Mac shop once, to get an ethernet to USB cable, and they didn't have one but the idiot there told me that any such adaptor would work, even the Windows-only one from Dick Smith. So I bought that and it didn't work! Surprise surprise. Luckily I could take it back, since I could repackage it to look like I'd never opened it. The computer shop at uni is also one of those official Mac resellers, but they don't offer any proper support to my knowledge. They DO have the cable, though. Just I won't be able to return it if it doesn't work.
And the computer itself was secondhand, from the university's IT department. It was surplus to requirements. I don't exactly want to take the computer to a computer shop, since it's fucking heavy.
I've already identified the problem anyway. Either the ethernet port/card is rooted, or the motherboard has catastrophically failed somehow. This is why I need an ethernet-to-USB adaptor, so that I can isolate whether the ethernet is causing the problem or not. If it's not, then fuck it, I'll just get a new computer. I need a laptop anyway.