Question : 400 error with the message that the request has used an invalid verb

When trying to go to a certain website one person is seeing a message 400 error with the message that the request has used an invalid verb, but everyone else can get to the website. They have tried using IE 8, Firefox and Mozilla with no luck. When they use Opera they have no problems viewing the site. Suggestions on how to fix the others? I did make them disable their anti-virus but still no luck. This is what they said.

I just placed my cursor over the hyperlink to your URL and I get a ScreenTip that says “blocked::http://www.website.com” but I have checked all my security settings both on Windows 7 and my antivirus, Kaspersky Internet Security 2010, and there are no settings that should be blocking it.  

Answer : 400 error with the message that the request has used an invalid verb

The client side Antivirus software is scanning the headers and,
apparently, re-constructing the headers inappropriately.

Put a client side exception for the website in place and report
.