If Windows is installed to and booting from an MBR-partitioned disk, then that means that Windows is booting in BIOS mode. On most computers, it's very awkward to switch between boot modes, so if Windows is booting in BIOS mode, it's best to install Linux in BIOS mode, too. My suspicion is that you have instead done an EFI-mode installation of Ubuntu. There are at least two ways you can proceed:
- Wipe your Ubuntu installation and re-install in BIOS mode. The trick will be to boot the installer in BIOS mode. The problem is that different EFI implementations provide different controls for how to set a CD's boot mode (BIOS vs. EFI), so I can't provide step-by-step instructions for this; you'll just have to study your firmware options or the computer's/motherboard's manual.
- Boot your installer into BIOS mode, or use a BIOS-mode rescue system such as SystemRescueCd, and use it to install a BIOS version of GRUB. This will go best if you create a BIOS Boot Partition, which is a ~1 MiB partition in which GRUB stores some of its code. (You can carve it out of your /dev/sdb1, which will be useless if you follow this route.) There are plenty of online guides describing how to re-install GRUB to a non-booting system, but I don't happen to have any URLs handy, so you should do a Web search.
Note that Linux is perfectly happy to boot from a GPT disk in BIOS mode, although Windows can't do this. Also, some firmware implementations have problems with this setup, so it's conceivable you'll run into difficulties if you try to follow it. (These problems can usually be overcome by setting the boot flag on the MBR's 0xEE partition using