"Access denied" for every connection attempt after some time

Hello,
I installed Vista about a month ago, and since the beginning I have this
strange issue: after some hours (between 10 and 30 approx.) my computer
is not able to create any TCP connection for from 30 to 180 minutes.
I don't know the cause of this problem but here are the symptoms:
- if I try to open any TCP connection, for instance my browser to
www.google.fr, I get "access denied error"
- active connections still work, e.g. IRC or Webradios are still up and
running
- my computer is accessible, for example if from another computer on the
LAN I try to access the FTP server which is on my computer, there is
absolutely no problem.
To sum up, my computer cannot establish a remote TCP connexion.

I am using Windows Vista Professional, which I had with the MSDN
Academic Alliance between my school and Microsoft.

I hope you will be able to give me some leads, or better, a solution!

thanks

Benoit D.