Eh, I have a tendency to occasionally try a bit too much. Usually the results of, uh, 'slightly overestimating' my abilities aren't irreversible, but...
I didn't quite think a manufacturer update program would go and gut the whole thing.

Ah well, live and learn. Replacing the hardware - in the end, pretty much everything except the floppy, CD and DVD drives - was actually easy enough if a little time-consuming, but the new SATA-compatible motherboard doesn't want to talk to the old IDE hard disk which means swapping that thing too which means a Windows re-install which is by all accounts better done by professionals, to whom we indeed turned the matter over today. They said it should be done by Monday.

A system upgrade was planned anyway, so we ended up doing it the hard way then. We may actually have gotten off with less expenses than buying a whole new comp would have involved, but a lot more running around and hassle to compensate.