start Minecraft, small window opens saying "Unknown Error"

I just purchased full MineCraft, downloaded on CPU, start game, but the only thing that is happening is a small window opens up saying "Unknown Error" & i can click "OK", that is all. I have downloaded and played the Demo on same CPU. The only difference is that I was using a different email / account address.
Are there any instructions on how to fix this issue? Thank you very much