My contact us form isnt working....

[WarrenSRQ]

I am not the greatest web designed in the world, but working on it, long way to go. I built my site using frontpage and built a form in it, the form is up and running on my site at www.silentdealer.com/contactus.htm and it just gives me 404 errors, and I have no clue why/

ANY HELP WOULD BE APPRECIATED!