Everything is OK now.

Although I am sure fdisk /mbr would have worked, I went the more
complicated route by building myself an Ultimate BootCD for Windows,
booting to that and running the mbrfix program. First I uninstalled
XOSL to be sure I could re-create my problem, which it did. I then ran
the program called mbrfix. This offered several options. I chose to
repair mbr for DOS/W2K/WinXP. The later version of this program also
works for Vista.

For all you lurkers out there, Ultimate BootCD for Windows is a very
handy set of tools. It also comes in a DOS-only version you can use to
create a bootable CD. Windows 2000 detected some change in the
hardware and wanted to reboot soon after coming back up. After that it
was Bob's your Uncle (or maybe Antoine's our Uncle would be better).