How can I boot, when forgetting password?

What you can do in OSX is put a system DVD into your drive and start your computer while holding the key "C" - computer will boot from system DVD. (you can also hold the option button and select the DVD)

It will show up the language selection etc... and when it is ready for new installation you will see in the bar at the top utilities. Then select "reset password" and follow the instruction : choose the account type in new password and restart the computer. DO NOT CONTINUE installing new system

In case you do not have system DVD's there is another option. In case you need it I'll send you how to do it.