I had great success using pskill from www.sysinternals.com (at least once I forced an unplanned reboot). I've also heard good things about prkill but I have not yet had a chance to evaluate it.
this might help, i'll go and test it right away, thanks!

Put the service start-up to disable and reboot. Usually, the service don't load!

Or just plain delete the service from registry! (I think their a tutorial on AO how to do it)
i already mentioned that that's a possibility, but like i said, i'm wondering if it can be done from within windows.

i know which services are needed to run the system, but like A/V software, this isn't needed, and still you can't kill it.

Do you have a specific process/need or you are just wondering?
several, but not really, i was just wondering, like i said, the A/V is one of them (this is because i work at a repair center, and when i boot up a system from a customer, often so much junk is automatically starting up, that we can't normally test the system.)

There is a way to kill system proces...

you can do it from cmd promt (run it as admin)
now you have to us at command... that command is used to shedule apps with localsystem priviledges.

I belevie that right syntax would be:

>at hh:mm /interactive taskmgr

where hh:mm is time in next few minutes so you don't have to wait long.
run at command again with no argumets to check if the shedule is ok

when time comes you will see task manager pop up, and this time you will be able to kill almost all proceses that usualy give you access denied.
like i said in my second post, even when you run the taskmgr as system, (this is with the method given by you).

b.t.w., did you try killing explorer.exe and restarting it again with that taskmgr? you will be logged in as system then, this means more keys in the registy, access to any file, you name it, it's possible... it's a nice trick