How are you trying to install FCP? You can't just copy it, Pro Apps need to be installed to work.
When you got your original computer with the pre-installed FCP-7 weren't there discs included?
You would be best served to install from them.
Also, if you are running FCP-X there are specific installation guidelines to have both FCP-7 and FCP-X installed on the same computer:
See this document:
Final Cut Pro X, Motion 5, Compressor 4: Installation best practices - Apple Support
about half way down the page.
MtD
Also: If you are doing a clean install of FCP-7, then you should use
http://www.digitalrebellion.com/fcsremover/
from Digital Rebellion (free) to remove the traces of your previous attempt to install.