Win 7 will install just as fine on win 7 unless you had another AV on the PC previously and did not remove it correctly or you have java script issues. Make sure all PCs a different name.
Best to contact Customer service Chat and see what they say.
Kindly provide us with the error that comes during activation , if possible a screen shot would be good.
Also, let us know if there were any McAfee programs on this PC before you installed the new one