Why am I getting this error every time I post?

[Valpollicella]

I write a post, click 'Post' and then the following comes up.

I can't see the message (even though it's posted) on the thread and have to click on Active to go back.

IE7 (I think) and desktop.

Ta MNHQ

Sorry meant to say the post button once clicked becomes greyed out (can't be pressed again)

And it just happened whilst posting the above as well

I'm having problems too. When I click post a box comes up on the screen telling me I've tried to download something.