Security :: ValidateRequest, And Basic HTML Failure?

Here's the situation. I have an aspx page that is designed to receive a POST request with some XML values, parse the XML, grab the relevant items, and write them to the page. The problem arises when I try to launch the page using the POST request. When I launch using Fiddler, building the request manually and just pasting the XML in the body of the request everything works fine and dandy. When I launch the page from a basic HTML form, however, things don't go so great. The HTML form that I'm using looks like this:

[Code]....

When the page loads I get the error:

A potentially dangerous Request.Form value was detected from the client Everything I've read so far has told me that the solution is to add ValidateRequest="false" to the page directive in the top of the .aspx file, or in the pages element of the web.config file. But neither of these work. Afterwards, I still get the same error. Any idea what I need to do to make this work?