I had this experience recently with my new Win7 install.
I installed Win7 on a new HDD, then plugged in my old XP HDD. The first time I turned the PC on with both HDDs installed, XP was the OS that loaded. I was actually surprised that it booted.
If you do install HDDs with different OSs, your initial startup screen should bring up an option along the lines of [Boot Menu - Fxx], where Fxx will be an F-key.