I need to authorize using admin password that I do not know

Hi,
I received a second hand computer running on Vista from brother as he is now using a laptop.
Seems like the system is running fine, and I thought of that until I received a pop up.
I have right clicked my computer and clicked on manage, and the pop up that is asking me to login using the administrator.
Now, I do not know the admin password and my brother knows nothing about setting up an admin password before.
How can I get through this?
Thanks!
