Windows 7 comes with one license. If I understand you correctly, you are trying to install Windows 7 Professional on your new computer, but you are unable to because the license is already used on a different computer?
If this is the case, you can call Microsoft and explain that you are no longer using your old computer and wish to activate Windows 7 Professional on your new computer. However, if your old computer has an OEM version of Windows 7 (meaning your copy of Windows 7 in permanently tied to your computer), you will not be able to transfer the license to a new computer. Here's how to contact Microsoft for activation - http://www.sevenforums.com/tutorials/18715-activate-windows-7-phone.html
Microsoft Windows Outreach