Urgent problem

Hey everyone, having a major problem with one of our computers.
We just installed comodo firewall on this computer and about half an hour later we are having an error message come up.
“Windows cannot access the specific device, path or file. You may not have the appropriate permissions to access this items”
This happens whenever we try to open a program such as comodo firewall or the command prompt. THe computer is pretty much useless now and we can only access folders at the moment but nothing else. We have not installed anything else after installing comodo firewall or changed any settings.
OP:windows XP professional
In need of dire help since there are a lot of important documents etc.
THanks all