Since the last response, I have done the following:

1. Updated the BIOS (to 10.14)
2. Reinstalled WinXPSP1 over itself
3. Uninstalled the memory stick & USB devices from both the Disk Drive & USB sections of Device Manager - tricky, since the system freezes when a stick is inserted and the device doesn't show up when it isn't.
4. Done some swearing.

Result? - nothing. Are there any third party USB memory drivers for WinXP?

Thanks,
John