Following up on the last post, I've made some interesting progress on the constant rebooting problem. It's still intermittent: sometimes the machine stays up for ages, sometimes it reboots over and over. During one of these moody phases, I noticed a flicker of blue and white on the screen before the system reset. Could this be a blue screen? I hadn't seen one of them on the machine for ages.
Digging around deep in the XP system options, I found a setting for "Restart after system error" which is set to true by default. I switched it off, and went back to work. After an hour or so, the machine flicked up a BSOD, with the irritatingly cryptic error "Page_fault_on_nonpaged area". That is, the reboots have been generating error messages. But those redmond based idiots have the default setting to reboot the instant the error is generated, so unless you have a photographic memory, you'll miss the error entirely.
This means that my initial conjecture about a duff power supply is probably barking up the wrong tree. Some rooting about on the internet suggests that the most likely causes are (a) a newly installed device (b) some duff memory (c) some disk problems. Since I haven't put anything new in the system in the last few weeks, that suggests (b) or (c) are the most likely culprits. Since I installed some new memory earlier this summer, it's possible that that is the culprit. Or could be some bad sectors (do they still exist? My knowledge of computer architecture is rapidly becoming obsolete). So, after the next hang up, I'll swap over the memory chips and run some serious hard disk scanning on the offchance it's a fixable bug. If it still breaks, then I can try removing one or both of the memory chips to see if that helps to fix it. No solution yet, but at least the suspects are a little narrowed down.