The firewall would get turned off by wither infection or another security firewall had been installed or file corruption.
In the Security Center what does it say you are using as a firewall? Can you enable it? What security programs are installed on your computer?
McAfee Security Suite? It probably turned off the windows firewall and used it own when you uninstall sometimes it asks to re-enable the windows firewall and you may have not checked that option.
So if its working is everything ok?
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