Crashes computer on user switch


I seem to be having a problem with the firewall (I would assume, although it could be the antivirus. The problem arose after the installation of both). It works perfectly fine on my settings, however the computer crashes to the point that the screen turns pitch black and the green “on” light turns orange if I switch user and then try to log out of that user, only option is to restart.

As this has never happened prior to the installation of the two products, I have to assume that they’re some how the cause. I can’t imagine that it would be the antivirus as it only runs on one username. It’s a great set of programmes as my brother uses them well on the computer upstairs (although if I log onto my settings while his are still logged on, the firewall crashes), so it seems it’s something to do with this.

Does anybody have any ideas what it could be and how to sort it?