Nope, WordPad is merely what comes with WinXP, and is not MSWord.
No legal free copies of MSWord are available.

You need some type of MSOffice or MSWorks, both of which are pretty cheap in their basic form.

As a simple workaround, merely use (CTRL+A ) to select all your text, copy it (CTRL+C ) to the clipboard, and paste it (CTRL+V ) into your Word processing program - WordPerfect in your case. Then spellcheck it, and correct any errors.
Repeat as required - not the cleanest of solutions, but this will work

Alternatively create the text of your reply in WordPerfect, and use the above to transfer the text of your reply to this or any other forum, or any new email.
Just be aware that any formatting you use won't work, so stick to basic text, and tidy it up before you post or email it.

Edit: Obviously your final paste should be to your email, reply etc.

Yes, this site does use some form of Java, which I don't trust so I use the approach in the previous paragraph above to get around this.

If you want to look at other spell checkers for automatic email spell checking, then you could try the suggestions at the bottom of my original link.
Can't vouch for them, as I've never used them, but you may well come up with other options if you google for it.