1st The firewall and anti virus should always be running dont turn them off or close them out they should always stay in the tool bar at the bottom right.
Now about the limited account issue. Just right click on the app your trying to run and click RUN AS then choose your ROOT/ADMIN account then enter your ADMIN password you have to have a password though. Post back
see now viruses/Trojans cant execute themselves without a ADMIN/ROOT password.