Change boot order for Wubi
The Wubi Guide on the Ubuntu wiki (manual uninstall section) gives some hints on how to edit the Windows Boot Manager boot order in Vista and XP.
In Windows Vista you can either use EasyBCD or modify the boot menu via Control Panel (Classic View) > System > Advanced system settings
; then choose "Settings..." under Startup and Recovery.
In Windows XP, edit C:\boot.ini.
According to this Wubi HowTo, the default OS can be changed in Windows 7 by going to Control Panel > Advanced System Settings
, and in the Default operating system drop-down selecting Ubuntu instead of Windows.
Click start and run msconfig(in Windows). Look for the Boot tab and change the order of the OS. I can't remember what it looks like its been a while, but its pretty self explanatory.
Good Luck and happy Ubuntuing!