Receiving the "BOOTMGR is missing" message indicates that the windows bootloader was successfully loaded by GRUB, but found an error in its own configuration. The first step is to boot using the Windows 7 cd, enter the recovery console (right at the beginning of the installer), and type the following into the console:
This will fix the windows bootloader files without altering the MBR. More info about the bootrec tool can be found here: Use the Bootrec.exe tool in the Windows Recovery Environment to troubleshoot and repair startup issues in Windows. Reboot and see if Windows will load from GRUB.
If that does not work some people suggest booting again from the Windows installer cd, and doing a repair install on windows. This will most likely overwrite grub in the MBR so ONLY windows will start.
If windows now boots, simply use the Ubuntu CD to recovery boot your existing Ubuntu install. Once it is started run the grub tool again to restore grub to the MBR, so it can start either Linux of Windows. Replace /dev/sdz with the device of your boot hard drive.
sudo grub-install /dev/sdz
More details about this process can be found in this very well written HOWTO: Recovering Ubuntu After Windows Install.
Hopefully both systems will boot from grub now. If not your problem is more complex, and some more answering might be in order.