MacBook with Windows Hard Drive

Ok, so my hard drive in my Macbook went bad so I took my old XP laptop's hard drive and put it in my mac and it worked! Now my friend and I need to get the OS (which I don't know because it was given to me). It's a Macbook Core 2 Duo and any advice and help will be great. When I start the computer now it goes to the prompt from Windows asking me to put it in three different Windows boots but if you click them it just restarts the computer. How can I make this work perfectly with the Mac operating system and make it run like it did?