You have a classic setup to get bitten by http://en.wikipedia.org/wiki/Dead_key issue. This is especially easy on Windows which keeps the keyboard layout state per thread/window. So many people are unaware that when the new window pops up the keyboard layout may have been temporarily changed for the duration of this new window being open. In all cases like this that I've personally assisted it was always caused by careless change in keyboard layout, keyboard driver, physical keyboard or the way the key presses are transmitted through the various "remote desktop" tools.

Like, are you positive that nobody in your household had recently started learning e.g. Spanish or stopped learning German?