I followed the procedure using EasyBCD and everything went fine. After having worked a bit under XP, I rebooted. XP booted up to the display of the desktop (where I can see my icons) for 1-2 seconds and then immediately switched to the Vista boot. Just before the Vista boot, I could spot for a split second a small blue screen in the upper left corner similar to the BSOD.

The xp repair-install did not solve anything.

This is the umpteenth laptop I've installed with dual-boot (XP installed on pc with pre-installed Vista) and never ran into this. I would like to avoid reinstalling xp completely.

Solved. I booted into xp Safe Mode successfully, did nothing there but reboot normally. That's it.

Thanks for supplying the answer so that others could benefit from it