Mathematica misinterpreting input from keyboard

Mathematica is misinterpreting input from the keyboard, as if I had chosen some other keyboard layout.

For example:

• [ is interpreted as @,
• ] is interpreted as uppercase D,
• £ is interpreted as o with a diacritical mark of some sort, and
• ( is interpreted as uppercase H.

In fact the behavior is even weirder: if I type two open parentheses, then the response is to first display an uppercase and then to insert an uppercase K in front of it (not even at the location of the blinking cursor). Also the behavior sometimes disappears, so that [ indeed produces [.

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You are possibly missing the special Mathematica fonts. You might need to reinstall the application, or the fonts themselves, depending on your OS. –  Verbeia Jun 25 at 21:01
You can download the fonts from Wolfram Research, Inc. here. –  Sjoerd C. de Vries Jun 25 at 21:11