If a partition on your hard drive is bootable, that means that there is an operating system installed on it. When your computer starts up you should be prompted to choose which operating system you want to boot into. If you're not being prompted, you can manually tell your computer to boot off of another partition using the System Configuration utility built into the Windows operating system.
Click "Control Panel."
Click "Administrative Tools." From this folder, open the "System Configuration" icon. This will open the Microsoft System Configuration Utility (called MSCONFIG for short) on screen.
Click the "Boot" tab. Highlight the name of the partition you want to boot off of and click "Set as Default." Click "Apply."
Restart your computer. Your machine will now boot off of the selected partition on your hard drive.