Hi,
Please clarify/follow the steps bellow.
Did you had the McAfee in the other two systems previously or is this the first time your try to install?
Did you use any other antivirus software in the system?
Steps can be tried:
Try to verify the subscription in the second pc (Right click on the M icon in system tray->click on Verify subscription)
Try to uninstall the McAfee thorough the Add and remove program and run the McAfee removal tool
http://download.mcafee.com/products/licensed/cust_support_patches/MCPR.exe and then try to install the McAfee in the second system
Regards
Bala