Ok, I understand that. But the one and only thing which I have changed is the mainboard. And I would say if the installation had a problem with it, it would have cancelled at the first phase of it and not during the final steps. I mean the mainboard is quite an essential part. It never had a problem with the other hardware, which is not new and had had this Windows before.
I have a driver cd from the mainboard, but it is not bootable and has just drivers for onboard sound, gfx & lan.
Can it be that it was just a bit dirt, shaken loose in the cdrom drive during the mainboard change?