I believe you, it's just damned weird, obviously many people have done it without this issue... Although I've never seen this happen before when copying into a vb text box perhaps some extra formating is being copied over for you? Try copying and pasting the code below... if that doesn't work try copying it into a text editor like notepad then re-copy from the editor and paste- that will remove any extra formatting.
Code:
<div align="center" style="margin: 5px 5px 5px 5px;">
<table class="tborder" cellpadding="6" cellspacing="1" border="0" width="100%">
<tr>
<td class="thead" style="text-align:center"><iframe width="100%" height="600" frameborder="0" scrolling="no" marginheight="0" marginwidth="0" src="http://maps.google.com/maps?q={param}&output=embed"></iframe></td>
</tr>
</table>
</div>