I have an MSI Wind on which I run both Windows 7 and Ubuntu. I rarely use the latter but I like to keep it around and with a 320GB hard drive its affordable. I have used both the Windows BCD loader and GRUB as my bootloader at various times (and for those using BCD I highly recommend EasyBCD as the way to configure it). Most recently I've been using GRUB.
Yesterday I installed Windows 7 RC and my boot sector got clobbered by BCD. I had a bit of a had time getting things working again. I first tried using EasyBCD but this time when trying to boot Ubuntu I ended up at a GRUB prompt, and I could not access my Ubuntu partition (more precisely, while "root (hd0,2)" worked, "kernel /vmlinuz" threw errrors and I could not boot).
In the end I got things working using the Ubuntu live CD and reinstalling GRUB. However, it required more than just doing a grub-install - for some reason the BIOS drive mappings seem to have been messed up. I did a web search and I found a lot of people have had this problem and very few found the solution so I thought it was worth mentioning what I did to make mine work in the hope it is useful to others.
In my case my primary Ubuntu partition is the 3rd partition; i.e. /dev/sda3. After booting the live CD and opening the terminal, here is the set of commands I used to get things working again:
sudo mount /dev/sda3 /mnt sudo mount -t proc /proc /mnt/proc sudo mount --bind /dev /mnt/dev sudo chroot /mnt grub-install --recheck /dev/sda update-grub
Without the mounts of /proc and /dev, /dev/sda won't be found after the chroot. The --recheck option is necessary because somehow the BIOS drive mappings have gotten confused. It's possible the update-grub step is not necessary but I think it is safest to rebuild the grub config; I read at one web posting where someone omitted this step and after rebooting was left at the GRUB prompt again.