Can not turn on firewall?
Try using this tool, this is the onecare firewall repair tool.
Save the file first before running. If you're using Windows Vista, right click on the file, run as administrator.
http://www.microsoft.com/downloads/details.aspx?FamilyID=7ee728a2-3c67-4a12-8d7e-954e86233206&DisplayLang=en
For further steps, you can refer to this KB article.
http://support.microsoft.com/kb/913786/en-us