I have installed Ubuntu 11.10, Linux Mint and Windows 7 in my system. I tried to set Windows 7 as my default OS but Linux Mint remains as default. I tried to change the order through StartUpManager in both Ubuntu and Linux; but no use. What can be done?
Find out which OS last installed (overwritten) the boot loader and configure default from there, using GUI (which you tried already) or config files.
Judging from the fact that Linux Mint is default, boot loader was probably installed from there. Mint is Ubuntu's clone, you might use the following page for info: https://help.ubuntu.com/community/Grub2#Configuring_GRUB_2
By the way, they're both Linux, not "Ubuntu and Linux" as you wrote. :)