Most Windows applications use the ANSI character set to display text. The actual character displayed depends on the font selected.
Before Windows and GUI programs existed (in the DOS days), special line drawing characters were used to "draw" simple boxes and frames in standard text files. These special characters were a part of what was called the OEM or IBM character set.
Windows (True Type) fonts do not usually support these line drawing characters. These characters will usually be replaced by some strange symbol.
However, some fonts (like Courier New) support both ANSI and OEM character sets. By selecting the DOS/OEM Char Set option (from the View menu) the OEM character set will be selected (if available), and your lines and boxes will be drawn correctly.
Note that this will only work if the selected font supports the OEM character set. If it doesn't, this option will have no effect. For example, an OEM character set is available for Courier New but not for Courier.