ASP.net adds very handy functionality by default, in that it throws an error if your form contains any html on it's submission.  Handy for html injection techniques, but not if your writing an entry form for techies that requires it.

To disable this validation add the following code to your aspx pages, just remember you've done it and the consequences of doing so.

<%@ Page Language="VB" validateRequest="false" CodeBehind= etc etc etc %>