I'd make this a comment, but it would be too long. I'm not sure if it's going to be a solution, or simply a guess, but here goes...
I'm guessing that you have installed grub on sdb, but you are booting from sda. When you install grub, there are two things you need to do:
1. Install something in the master boot record (MBR) of the drive that is set to boot by your BIOS. This is sda on your system. This disk's MBR apparently has only the windows boot loader, not grub.
2. Tell this boot loader where to find the grub menu file, normally in
/boot/grub/grub.cfg of the last Linux partition installed. Note that if the MBR code doesn't know where to look, it does not matter in the least what is in menu.lst, because it will never be seen.
The easiest way to fix the problem is to download and burn a live CD called Super Grub Disk. You can find a lot of discussion on askubuntu, but it's pretty self-explanatory, I think.
The other way is use the command line, and run grub-install, telling it where to put the MBR code; you would need to mount the disk to do this, and run it from a live CD. The command would be something like:
sudo grub-install --root-directory=/location of root directory /dev/sda You would need to specify the correct location for the root directory of the distro for the grub.cfg file, unless it's run on the live system (I've never tried that, but it may be possible, though probably not recommended). If you want to go this route, I'm sure you can find step-by-step instructions.
Once the MBR is set up, you can run
sudo update-grub to update the menu file.
You may also be able to use the BIOS setting to choose which OS to boot, by picking either disk 1 or disk 2. This should work, although I've found it to be problematic in the long run, because grub will eventually get installed on disk 1 by mistake.