Comps being "hacked into"

Hello all,

I have something of a situation on my hands and would appreciate any input. There is a forum I have frequented for many years and over time it has grown so big to the point that old sk00l vets such as myself formed a separate smaller forum just for us on a separate site. This forum was set up by one of the other vets.

Now, it appears that the guy who runs this forum is obtaining information from our computers. It seems he is able to view what other sites we have been going to, in particular other forums. He has even been able to get passwords for some forum accounts... (perhaps via cookies?) Now don't get me wrong, this is all being done in good humour and everyone has complete trust in this guy, but I myself would like to know what steps I can take to stop someone seeing what websites I have been to, my login information etc.

What (free) programmes can I use to protect myself? I have windows vista, with windows defender and AVG. I would appreciate any help.

Also, when I log onto a website, can that website see what other programmes I have running?

Thanks for any help.