I need to send an email to people, which contains a question, and two
radio buttons so they can select Yes or No. I (and the audience) use
Outlook 2003 or an IE interface to access the email. The person
readint the email will hit "reply", select their answer and then send
the email to me. I want their chosen answer to remain selected when
they send me the email, of course. There will be other questions in
the email, with free form answers, I am just asking about the radio
buttons.

I assume I can do this by sending an HTML email - what is the HTML code for it?

Depending on HTML email to do voting is a bad idea, as there are a
number of us that prefers straight text email and thus would not be
able to respond to your radio buttons. Having them choose a link to
click on and counting that is better.