After another day of work, my PC is more or less OK again. I did however need to reinstall everything.
First I had to reinstall XP over the version that was already there, but not accessible. I then backed up, my e-mails and documents to a second partition. The windows version was not usefull for anything else but this, so again a format of that drive and XP reinstall. That’s the 5th in 3 days, believe me, I don’t want to see an OS install in a looong time.