This was a bit stupid. Exchanged the mobo first, wanted to start the upgrade to XP Pro but just couldn't resist to try the Serillel option first. That really was my "Preparation". Changed the BIOS settings (autofinding the HD) and then of course you know what happens: the BIOS will write something to the drive, you're probably too fast typing something on the keyboard, it will interfere with the writing process and you'll end up having garbage on the drive.
I will follow your kind suggestions and make a clean start in the right sequence. But for the time being I wil try to restore my old files as it's worth investing some time instead of rebuilding the data store.
Thanks again,
Roel