If it happens just with Shift+S, then it's probably the result of an
accidental misassignment. Assuming you're using Word 2003 or earlier, choose
Tools - Customize - Keyboard.
Set Categories to All Commands. Set Commands to Style. Look at Current keys:
Ctrl+Shift+S should be listed there. Is Shift+S listed, too? If it is, then
click on Shift+S, then click the Remove button. This will release its
assignment to Style, and return it to typing S.
If it happens with other letters, too, then it's possible that a) your Ctrl
key is stuck, either physically or for some software reason, or b) your
keyboard is dying/dead.
In the former case, you usually can get Ctrl to turn loose by tapping both
Ctrl keys (assuming you have two) several times.
