Using the built-in Sysinternals utils:
taskkill /S hostname /IMor
sc \\computername queryex TrustedInstaller...I could have been able to terminate that task. According to some internet records if I had done so, my server would have restarted without a hitch. Luckily enough, after entertaining me for more than one hour (-meanwhile actualy doing nothing-) this damn server finally restarted by itself! After the reboot it installed the updates and got ready in less than 5 minutes.
taskkill /s computername /pid /f
So, in case you are here because you are in the same business as I was, now you'd better have some coffee and wait patiently instead of roughly interfere.