The problem is that your BIOS only boots your first hard drive (sda), which has only the Windows Boot loader installed. The best thing you can do is to get a LiveCD, and enter the grub prompt (open a terminal and enter grub as root)
grub> find /boot/grub/stage1
The result of that (e.g. hd1,0) is the partition and hard drive where your grub installation resides. (If you have not created a separate boot partition, it is probably (hd1,0) i.e. first partition on /dev/sdb )
grub> root (hd1,0)
grub> setup (hd0)
This will install grub to your MBR.
More information can be found in this