A family of Microsoft word processing software products for creating web, email, and print documents.
In order to be able to select a portion of a word, you must clear the check box for "When selecting, automatically select entire word" at File | Options | Advanced: Editing options. But that doesn't explain runaway highlighting such as you describe unless your mouse button has a hair trigger: double-clicking will select a word, Ctrl+click a sentence, and triple-click a paragraph.