You may have noticed that Windows doesn't use Ctrl+Alt as a keyboard shortcut anywhere. (Or at least it shouldn't.) If a chorded modifier is needed, it's usually Ctrl+Shift.
That's because Ctrl+Alt has special meaning on many keyboards. The combination Ctrl+Alt is also known as AltGr, and it acts as an alternate shift key. For example, consider the German keyboard layout. Notice that there are three keyboard shift states (Normal, Shift, and AltGr), whereas on U.S. keyboards there are only two (Normal and Shift). For example, to type the @ character on a German keyboard, you would type AltGr+Q = Ctrl+Alt+Q. (Some languages, like Swedish, have a fourth state: Shift+AltGr. And then of course, there's the Japanese keyboard...)
Most international keyboards remap the right-hand Alt key to act as AltGr, so instead of the finger-contorting Ctrl+Alt+Q, you can usually type RAlt+Q.
(For reference, here are diagrams of several other keyboard layouts, courtesy of my bubble-blowing friend, Nadine Kano.)
Sometimes a program accidentally uses Ctrl+Alt as a shortcut modifier and they get bug reports like, "Every time I type the letter 'đ', the program thinks I want to start a mailmerge."
[Raymond is currently on vacation; this message was pre-recorded.]