I'm back in XP Pro now. All is well, although I have to be here to tell it to start that, rather than Win 7.
I suspect you are well aware but you can change the defual OS in boot.ini.
As for Windows 7 - I'm obvously unlucky in some way. Went for the 64-bit version - it won't upgrade from XP so it's got to be a clean installation. OK, it's going on a secondary partition so what harm can it do thinks I.
The installation looked to be going OK but after the first restart I got a BSOD saying that it didn't support my multiprocessor or somesuch. In the end I had to boot using an old recovery disk and then had to manually get rid of the Vista bootloader stuff. Still, had nothing else to do for that hour!