TinySpell is a simple spell checker for Windows that you can use on any application that does not have a native spell checking feature.
TinySpell watches every word that you type, and whenever it detects a misspelled word, it notifies you with a beep, yellow tray icon and the correct spelling tooltip. You can then click on the tray icon for corrections.
TinySpell also keeps an eye on the clipboard, so if you need to check a web page for spelling mistakes, just copy it all, and let TinySpell do the rest.
The basic version of TinySpell is freeware, TinySpell+ has a few additional features at 10$.