[Martin CX]

It's not hard. Locate this in the proper registration-templates:

<tr>
	<td bgcolor="#DFDFDF"><normalfont><b>Send you an email when you receive a Private Message?</b></normalfont></td>
	<td bgcolor="#DFDFDF"><normalfont>
		<input type="radio" name="emailonpm" value="yes"> yes
		<input type="radio" name="emailonpm"value="no" checked> no
	</normalfont></td>
</tr>

Now replace it with:

<input type="hidden" name="emailonpm" value="yes">

That way all new users will receive emails when PMed, but can disable it if they please. Just do the same with the other preferences you want to fix.

I do the same (with time zones and other stuff) a lot of users get confused with the many options.