Nah, that wasn't the problem.
For some reason the BIOS didn't remember me setting SATA to AHCI which I only discovered later. Manual corrections didn't work (didn't found the fitting instructions since my board is relatively new), so I had to reinstall XP as second time. And doing that I discovered that I made a mistake setting up my drives, so XP was installed on a different partition than the one holding the MBR.
Also I had a short moment of terror when I accidently deleted a partition, but I got lucky since I had just copied the content the night before (*phew*)
My plans for Win7 are to install it to C (since it will be my main OS, and old habits die hard) and have XP on another drive/partition. But since now I've ended up installing XP on C again I guess that will take some tweaking, or even another install (if it is possible to install XP after Win7 ... guess that will overwrite the MBR which then will have to be repaired with Win7 afterwards).
But where would be the fun if it was that easy ...