Windows 7 not genuine build 7601

Hi there! I'm having issues with activating my Windows 7 product key. My desktop background has gone black with a small note at the right bottom corner with the message: Windows 7 build 7601 this copy of Windows is not genuine. I own a 3 year old Toshiba laptop (running Windows 7 Enterprise 64bit), the OEM product key sticker on my laptop has worn out, I used a key finder to find...