You can reactivate with OEM SLP usng the ABR program and Dell cert files. Personally however I would perform a complete clean install because it looks like the person who clean installed Windows 7 Pro didn't know what they were doing... See here: [URL]  
I've been given a computer in the following state: - Hard drive was reformatted with a clean install of Windows 7 Pro (same as shipped) which is now showing as not activated. - I was told that an OEM disc was used to reinstall Windows but it looks like a retail version might have been used instead along with what appears to be a generic key. - I do not have recovery media and I don't see a COA label on this model. The computer all original parts and it's my understanding that the activation key is BIOS based. Is there way to activate Windows on the spot without tracking down recovery media, backing everything up, and then reinstalling? I apologize if I've missed any critical information. Any advice is greatly appreciated.
OptiPlex 3020: Windows 7 Clean Install - Failed Activation