How do I reset my Mac OS X admin root password?

This is my first post on MacRumors forums and I am also a new Mac owner myself.

Unfortunately, I'm in trouble as I have just bought a 2009 Macbook Pro (running Lion) from a friend of mine for my fiancé who now has the computer living in Korea. We are having trouble figuring out the admin password to change the security setting to allow her to download programs other than the one's just approved by "Mac and Identified Developers".

Does anyone know of a way to change/discover the password without buying an Mac OS X install disc?

-Thanks for reading

 

Awesome.

Thank you so much! I will try this and hopefully the fiancé is savvy enough to follow computer directions.