certification and pretence sites

I was accessing a Microsoft security site when my browser (Mozilla) came up
with a message that the site c.microsoft.com.... could not be certified and
that I might be connected to a site pretending to be something other than
what it was.
>
Before this I had been to a site which called itself a Photo Story 3 forum,
but which I later realised was displaying only a page of the Microsoft
Photonewsgroup and was in fact inactive. But the "contact" button worked
and I exchanged e-mails with someone I assumed was a forum moderator who
"registered"me and I made a "posting" (which has since been removed from this
shadow site, although my user name has been given to another posting!)
>
To day I found my Photo Story 3 programme has been "disappeared" from my
hard disk! Thnakfully my photo data is intact and I've backed it all up. I
think I have been a bit careless, but what has been going on and what should
I do to protect myself? I've run my Norton Security and a couple of Spy- and
Adware
programmes and no threats have been found.
Jeremy