You're using the form builder which is perfectly sensible, though you have set the Successful URL for your form to a page that doesn't exist. Hence the form submits OK, then it tries to show your Successful URL and that's when you see then Page not found.
The fix for this is to change this Successful URL to point to a page that does exist on your site, or leave it blank so that it shows the standard Thank You page.
Alternatively, I think a better way to do a contact form is to use the Contact Email includelet on a normal content page. That's how it works on the demo site.
Joe - Voice Admin