This is how I changed my default boot option to my Ubuntu 9.10 , I installed my ubuntu through Wubi w/in my W7

1. Click Windows Knob ->Control Panel

blog.dpjonline.com , Start Control Panel

Start Control Panel

2. At the control panel window click “system”

www.blog.djonline.com , Control Panel Window

Control Panel Window

3. at the system window, click “Advnace Settings”

www.blog.dpjonline.com , Windows System Options

Windows System Options

4. click advance tab->settings

5. choose ubuntu