Debian 6.0 on amd64 and i386 uses the grub-pc (Grub 2) boot loader by default. It's complicated and there are still some problems with setting grub passwords. Changing to the simpler grub-legacy can be done with the following commands:
apt-get remove grub-pc dpkg --purge grub-pc apt-get install grub-legacy grub-install /dev/sda update-grub