If it's not the same model of motherboard, it will cause a problem if you are using an OEM version of windows 7. OEM licenses are tied to the original hardware they are installed on. If it's a retail version, it won't cause a problem.
When you start the computer, tap the delete key repeatedly. That will bring up the BIOS. Look for the advanced boot options. Then find the Boot Order, or First Boot Device. Change that to the CD/DVD drive. Put your windows disc in the drive. Save the changes, and exit the BIOS.
It will then restart, and see the DVD. It will then display a message to 'press any key.' Do so at that time. Here's a guide to get through the install process.http://www.buildeasypc.com/sw/windows_7/install_windows_7.htm