This workaround can only be carried out for an eligible system i.e. a system which contains a valid Windows XP, Windows Vista or Windows 7 OEM license generally most Dells which have a Windows COA affixed to the system or a full retail license of a previous version of Windows.
When inputting a product key from a COA (assuming you have not used the ABR program, this saves the need to call Microsoft for Product activation).
This workaround is intended for those who wish to clean install using their retail upgrade product key e.g. if they are replacing their hard drive or have securely wiped with DBAN and don’t want to waste time with a double install.
It essentially involves entering the product key via an elevated command prompt and activating via the elevated command prompt as it will be rejected if input via the Windows 7 directly.
Press the Start Button and in the search box type in CMD.
Right click CMD.exe and select run as an administrator. This will open up the elevated command prompt accept any user account control prompt.
To input your product key type
slmgr /ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx
Where xxxxx-xxxxx-xxxxx-xxxxx-xxxxx is your product key and then press Enter.
Select ok and then to activate type
This will activate your product.
Select ok and then close the command prompt. This solution taken from MyDigitalLife forums here and I have verified it.