Eli Bendersky's website Sometimes Windows runs system services which clog the CPU and/or memory, and generally interfere with work.
One of these services is cisvc.exe - officially a "memory monitor" process for the "indexing system" cidaemon. Well, today I found that cisvc and its friends were happily taking half of my CPU resources and made working on the PC impossible. Restart didn't help, and trying to kill the process results in an error that it's a system service which must not be stopped.
To disable this process and its brethen of the indexing service, minimize all windows and press F3. The "file search" window opens, where in the bottom left you can see that the "Indexing service is currently running". Clicking there brings up a dialog which allows to stop the process. Viola ! Back to work.