I'm no expert here, but this is how I do it, why re-invent the wheel.
As I understand it you have Windows and two linux distros.
When you install linux go to advanced and put grub on the first sector boot partition. I don't think you can do this with Ubuntu, could be wrong.
OK. When you have finished installing and reboot obviously Windows is the only one you will see. Now download and install Bootpart from http://www.winimage.com/bootpart.htm
Make sure bootpart.exe is on the root in Windows, i.e. C:\
Now go to command line in Windows and run Bootpart. You should see your linux distro there, should be marked 83*
Determine what number that is, then type
bootpart 2, (or whatever your 83 is) linux.lnx yourdistroname
bootpart 2 mint.lnx Mint 5
Press enter and reboot. Now you have a choice of Windows or Mint.
Works for me everytime. This won't mess up your Windows MBR and of course you can choose what is your primary system thru Windows.
Hope I have explained this OK, but there is more information on the winimage site.
If you have two hard drives even easier, I have Windows on the first drive and Mint+Fedora on the second. Both boot from Windows using the above method.