Firewall: Burnt down. Help!

When turning on my laptop (Toshiba satellite/operating system Windows 7) the McAfee firewall continually turned itself off. On investigating this further it was suggested that I uninstall McAfee to enable me to run anti malware software (mistake). Once uninstalled I was unable to reinstall or install any other software (notification on trying to run any software or reinstall McAfee was "a virus had been detected in the software and had been quarantined"). Further reading suggested I check the computer firewall. Upon checking I have lost the ability to modify this also (when trying to modify error notification message 'Error code 0x80070422' and also 'Error code: 0x643').

Any help would be much appreciated

Thanks

J