can't close certain processes

I'm using Windows XP Home Edition SP2, and for some reason, since recently, when i open the task manager, it wont let me close any programs under NETWORK SERVICE, SYSTEM or LOCAL SERVICE that I have been able to close before. Whenever it asks me if i want to terminate the process, I hit yes and it says "access is denied" I think it might have been ever since i installed an automatic update for Windows that this has been happening. I know these processes aren't required system processes and can be closed without affecting anything. I wanted to close them to lower memory usage when i play games. Any help?

the processes usually running under those headers are req. by windows. Kiling them increases the unstability of the OS which willl probably respawn them anyway when req.

The update from MS prevents thoses services being killed by certain types of malware/virii if i remeber rightly.

If you do want to disable extra services, then type services.msc in the run prompt and disable extra services which you dont req,