I get "operating system not found" on warm boot
i.e. ctl-alt-del / restart.

The system works just fine if started form power-on/off (cold boot).

Cannot understand why this would be the case. The problem
started after I uninstalled my firewall. I have tried since to
install/re-install again without any solution.

I have aslo done "fixmbr" and "fixboot" from
recovery console, but my problem still presists.