Have local admin privileges on Windows XP, but getting "Error terminating process: Access is denied". How to kill the process?

On one of the Windows XP machines I use regularly, there is a process that starts up periodically. I'd like to be able to kill the process – sometimes – because it occasionally runs when I'm busy doing something machine-intensive.

I've already tried dropping the process priority to "Idle" to mitigate the effects, but it isn't the CPU that's the problem. Rather, the process is very disk-intensive and no matter the process priority, it still causes significant disk thrashing when running, impacting everything else I'm doing at the time.

Using Process Explorer, I can find the process, right-click, and choose Kill Process, but I always get the message "Error terminating process: Access is denied."

This is not an operating system process, but third-party software. What might that process be doing to prevent itself from being terminated?

How can I kill such a process? Is there a way for me to modify the process's security or access control list (ACL) somewhere, using Process Explorer or another tool, so that I can effectively kill it?

The process could be starting with the SeShutdownPrivilege set to Deny (or not set to Allow) for you or the group you're in.

Fire up procexp.exe, find the process, right-click --> Properties --> Security tab --> Permissions button. You may need to run procexp.exe elevated to see anything.

Looks like I'm able to kill the process in question using Process Hacker, which, coincidentally, I learned about today from a SuperUser Community Promotion Ad:

One of the features listed is "powerful process termination". From the project page:

Process Hacker is a free and open source process viewer and memory editor with unique features such as powerful process termination. It can show services, processes and their threads, modules, handles and memory regions. [emphasis mine]

While I don't intend for to replace Process Explorer, this is certainly a complementary new tool worth keeping around, and I'm curious to learn more about it.

taskkill /f will do it!

What is the software? Sounds like it's running as a service. You'll get that error when you try to kill a process (from Task Manager or Process Explorer) that is running as a services.

at 16:44 /Interactive taskmgr.exe

This would add a job to the scheduler to run the taskmgr.exe at 16:44. Select your time about 1 minute in the future from current time. The taskmgr.exe would run with the System account which has additional privileges. See if it works.