Wordpad is what is known as a word processor. Its good for formatting documents that look good for humans. What you see on screen does not necessarily reflect the contents of the file.
Notepad is a text editor. It doesn't allow for any text formatting but that is exactly what you need when programming because things like italics generally don't mean anything to a compiler or interpreter. When you open a file in a text editor, you are viewing exactly whats in the file.
Neither Notepad nor Wordpad support all of the characters found in a bot file so you'll need to find one that does since the bot file contains TGA data for the skin and preview image. Most of the people here use Notepad++ but there are plenty of other options if you can't get it to work.
That said, you shouldn't get any error if you install Notepad++ by downloading and extracting the ZIP instead of the executable.