Http_referer

I go to post on a messageboard which i've never had problems with and now everytime i try to login or post a message i keep getting this message.

It's the same message on both Internet Explorer and on Mozilla Firefox, all the programmes are up-to-date, my firewall is the same as it always has been and every other site is fine.

Any suggestions to fixing this would be muchly appreciated.

Thanks

 

Is this a work computer, or a home computer?

 

It's a home computer, the site works fine on other computers and no one else has this problem.