I have a MacBook Pro and run Windows XP on it too much however I cannot remember how I can open the MacBook Pro with OS X on instead of Windows XP. How can I boot back to OS X?
Hold ⌥ on boot and select your OS X partition. This changes your boot partition once for that time you boot.
If you want to change it permanently, boot to OS X, open System Preferences → Startup Disk and select your OS X partition, or boot to Windows, open the Boot Camp Control Panel and select the OS X partition as the boot partition. Then, you will need to ⌥-boot to boot to Windows.
You have to Hold option key upon start up, two HDD will apperas, select the bootcamp disk or windows disk and you are done.