Can't change Windows Security Centre Firewall settings

The machine is a HP Pavillion a1350a running Windows Media Edition. User had problems installing McAfee Security Centre. Also, 'Windows has encountered a serious error..." messages continually created leading to a spontaneous reboot.

McAfee uninstalled via Control Panel. Windows errors tracked down to conflict with Windows security update KB908531 (reported conflict with HP computers) - this was uninstalled and fixed the error messages.

Installed Norman Internet Plus (firewall, adaware and antivirus) - installed ok and updated over the internet. Upon reboot, Norman is not running (but Adaware SE Plus is) - I suspect the Firwall is blocking Norman - Windows Security centre says:

No virus software installed - McAfee AntiVirus is turned off. The firewall is on, but cannot be turned off (or changed to th Windows firewall). Downloaded and run the McAfee removal tool which had no effect. Had previously run SpyBot, CounterSpy and Adaware and system is clean.

Is there any way to reinstall the Security Centre, or does anyone have any ideas how to rectify this aside from a repair or fresh install?