Control Panel->Security->Windows Firewall->Turn Windows Firewall on or off (listed in the left pane).

Windows Firewall should be listed using Services.msc but not MSConfig. If it's not listed by running services.msc and the above Control Panel option doesn't work you may want to run System File Checker sfc /scannow (with the space) and have your Vista DVD handy. Alternatively try System Restore, depending how far back your restore points go and how much you've installed/uninstalled recently.

John