After a Windows 7 virus cleanup, the Firewall would not start. It would give an error message 0x8007042. Found out the Firewall Service (and a few other services) were missing. The article below show how to restore the missing services and start the firewall.

http://social.technet.microsoft.com/Forums/en-US/w7itprosecurity/thread/5366225a-46e7-4d6c-a389-8bd18a5c3aad

Tags: Windows 7 | Firewall | Virus | Error | SOLVED | 0x80070424