If the hard drive that you just put in came from a computer that differs greatly to yours, then most likely it wont boot up sucessfully. Even if it did boot up, you performance will suffer greatly.
If you would like to try, we will need to know if it's an IDE, SATA, or another type. If it's IDE, then set the hard drive you want to boot from as Master, and other 2nd drive as Slave via the jumpers. If there both SATA drives, then go into the BIOS and select which hard drive to boot from first.