When I try to validate a local HTML file from Internet Explorer 6, I get this 
message:

Sorry, I am unable to validate this document because its content type is 
text/plain, which is not currently supported by this service. 

The Content-Type field is sent by your web server (or web browser if you use 
the file upload interface) and depends on its configuration. Commonly, web 
servers will have a mapping of filename extensions (such as ".html") to MIME 
Content-Type values (such as text/html). 

That you recieved this message can mean that your server is not configured 
correctly, that your file does not have the correct filename extension, or 
that you are attempting to validate a file type that we do not support yet. In 
the latter case you should let us know that you need us to support that 
content type (please include all relevant details, including the URL to the 
standards document defining the content type) using the instructions on the 
Feedback Page

***

I have checked the file for any errors, and I've seen none. I can, however, 
validate the page from a program like N|VU.