I would make a backup of the computer before the tinkering begins.
It's easy enough to uninstall Outlook Express, but what about your email and addresses? Are you willing to let all of that go? Uninstalling it will remove those files. Give this some more thought--I think you'd be better off uninstalling and then reinstalling the firewall software. When it is reinstalled it should recognize the programs you use, like Outlook Express, or if it doesn't, first time you use it you should get a firewall dialog box asking if you want to allow it access.
After reinstalling the firewall you'll need to download all of the updates that go with it. And if it is a Symantec product, then you may need to uninstall and reinstall your Antivirus at the same time (if it is also Symantec) because if they're not installed at the same time they don't always play well together.
To simply disable your firewall, look at the system tray in the bottom right-hand side of your screen. Click on the firewall icon that is bound to be there. Left click should give you a little menu of things you can do right there, right click will probably open the program itself. You should be able to disable it either way. You can disable it for a few minutes at a time or turn it off until you're through. Keep in mind that if you're online you're vulnerable to stuff your firewall is watching out for.