Well, if they are using Outlook 2003, just type
http://www.yahoo.com and
their client will have it clickable. Even in a text formatted email. The
only reason I can think it would be clickable is if you're using Wordmail
and you don't hit enter at the end of the url (and old bug which I don't
think happens in 2007). If it's clickable on your screen, it should be
clickable on theirs. So, the only question is: Do you want them to see
'Click Here!' or do you want them to just see something clickable. If the
latter, just type the url. If the former, Insert-Hyperlink. More or less
like I said in the first message.