Did Windows 7 come preinstalled on the system your trying to virtualize or was it a Full Retail version you installed? If it was preinstalled OEM version and not a Full Retail version then it is not transferable by its EULA! 
Also Manufacture specific OEM versions will fail WPA (Windows Product Activation) on the system it's transferred to!
If you can get the Migration Assistant in VMware Fusion to work then another option is to use VMware vCenter Converter.