I have purchased win 8.1 64bit and installed it on 02 systems, one system is working correctly while the second one remind us for activation. which is already activated on the same cd key as we installed on 1st one. I have to ask why this is happening, can we install win 8.1  on one computer or so many... let me getout of this please.

1 license, 1 computer. If you want to install Windows 8.1 on additional computers, you need to purchase additional licenses.