ASP.NET 1.1 has Request Validation. I guess what this means is that they look for suspicious data coming into a page from a form POST. If it's "potentially dangerous", they throw a HttpRequestValidationException.

To turn this off, you add this to web.config:

<system.web>

   <pages requestValidation="false" />

</system.web>

<pages> Element

You can also do this on a page-by-page basis (probably the preferred mechanism for a large site) by adding to the @Page directive:
<<@ Page ValidateRequest="false" ... %>