How do you change the default OS that GRUB loads?
In menu.lst, perhaps found somewhere beneath /boot, there is an entry: "default."
"default" is the index number of the title entry that you want.
default 0 title foo ... ... title bar ... ... title baz ... ...
In the above example, the default is "foo." If you wanted to change it to "baz," you would set default to 2.