TinySpell (freeware) can watch your typing on the fly and alert you whenever it detects a misspelled word. It can also check the spelling of text you copy to the clipboard.
After installation you’ll see a small icon in the system tray. Normally the icon is white but it turns to yellow when a misspelled word is detected. Clicking the yellow tinySpell icon, or pressing the hot-key (Ctrl+;), pops up a list of suggested replacement words from which you can select a correction.
You can set tinySpell to insert the selected word into your document or copy it to the clipboard (or both). tinySpell comes with an American-English dictionary containing more than 110,000 words.