Press the power button and hold it in for 15 to 20 seconds. That will shut down the computer, and allow a manual restart
However, I do not understand the password error message... so maybe you don't yet want to turn it off.
Did you use Add and Remove Programs to delete McAfee? If not, do that now.
Also, go to your BIOS and change any settings related to passwords, and read your manual about anything to do with passwords... As well as the McAfee site.
I assume this is part of McAfee normal security in their new Security Suite, but I detest McAfee for these gimmicks to much that I no longer use it.