1. Get a rescue disk. The Dapper CD should be ok for this, so long as it boots on your laptop (presumably it does) and you can get to a useful shell or the desktop.
2. Backup the MBR:
dd if=/dev/hda bs=512 count=1 of=/tmp/mbr.img
Save /tmp/mbr.img somewhere safe, maybe a Flash drive?, and then you can restore it with:
dd if=/tmp/mbr.img bs=512 count=1 of=/dev/hda
That way, if you do torch the MBR again, you can repair it using your rescue CD rather than re-install the whole OS, which will get boring quickly. Assuming that this fault isn't blowing other parts of the HD away.