Assuming you are referring to Internet Explorer: open one instance of Internet
Explorer, press F11 to have it switch to full-screen mode, then close the window
using the X in the upper right corner. The next time you open IE, it will open full
screen. As long as the last browser window you close is in full screen mode, it
will open the next time in that mode. We have been doing it that way at work since
Win98 and IE 5.x
If for some reason it does not remember the setting at some point, open a single
browser window, press F11 to set it to full screen, then hold down the Ctrl key
while you close it with the X in the corner.