Bill Gates
The real name of Bill Gates is William Henry Gates III. Nowadays he is known as Bill Gates (III), where "III" means the order of third (3rd.). By converting the letters of his current name to the ASCII-values and adding his (III), you get : 66+73+76+76+71+65+84+69+83+3 = 666.
Also : MS-DOS 6.21 = 77+83+45+68+79+83+32+54+46+50+49 = 666
WINDOWS 95 = 87+73+78+68+79+87+83+57+53+1 = 666
[Note] No explanation supplied why this +1 at the end. It would make more sense if there was a Windows 96.
Also note that the Windows logo contains 3 rows of 6 black squares.
Adolf Hitler
When we assign A=100,B=101,C=102 ... Z=125, then adding up the numbers of HITLER will be 107+108+119+111+104+117=666.