Use can use the DVD on as many computers as you wish, but you'll need a separate CD key for each additional installation. Microsoft's User Agreement/License allows installation on one PC per license (the CD key). OEM versions die when the original system to which it was installed dies. Full and Upgrade versions allow the OS to be transported to another computer, but it must deleted from the the earlier computer.
When you buy Windows, the valuable part of your purchase is the license key - THAT's what lets you install and activate Windows. You need a different key for each computer (except for a few special cases that you pay extra for).