Just install the grub bootloader on the Ubuntu partition rather than in the mbr (making sure that the Ubuntu partition is a primary partition).
You can then tell the mbr code to boot from the Ubuntu partition rather than the Windows partition and away you go.
(This is harder than it should be: all you need to do is to make the Ubuntu partition active rather than the Windows one. Unfortunately there is a bug in the Windows disk management utility which won't allow this so you can wither use the "diskpart" utility at the Windows command line, or boot live Linux system and use gparted, fdisk or whatever.)