Need to Reinstall, Have Activated Windows 7 on my PC, But lost the CD and Serial number

Edit: The answer was to find the Serial number first using a serial number finder utility. The possible duplicate was missing the additional condition that the serial number was missing.

I have Activated Windows 7 on my PC, But lost the CD and Serial number.
I need to reinstall Windows 7, I (hope) can borrow a Windows 7 DVD somehow, but how do I get my original serial number, I have the product key as shown in System Properties.

Thanks for any ideas

Answer

Product Key Finder from Dave Hope is quite good at finding what you're looking for.