Sorry, yes, forgot to mention that. It's XP Home, SP2.
I've looked on the ASUS website and come across that advice, but I'm a bit reluctant to flash the BIOS at the moment for the simple reason that it was working to begin with and it seems more likely to be a Windows issue than a BIOS issue. Of course, if I run out of ideas, it's certainly something I'd have to try - thanks for that advice.
A couple of sites have suggested isolating the power supply to drain any residual power remaining in the capacitors, rather than just rebooting it or switching off/on with the power still connected. It seems to have worked for other people who've experienced similar problems. To that end, it's been left overnight with the power cable out and the battery removed. I'm not really expecting it to work, but just waiting to hear from her.