What Windows Services can I turn off?

With one exception, my advice is to leave well enough alone. Most of the services running by default are either required for something you care about, or relatively benign. In normal usage, if a running service isn't actually used or needed, it takes up very little memory or CPU time. (If it does, you likely have other problems, such as a virus or an insecure connection to the internet.)

If you're trying to feel up memory, you're really better off just adding memory to your machine. In the long run it's cheaper and more effective.

Windows Messenger Service

The one exception? The "Windows Messenger Service". Please note: I'm not talking about the Instant Messaging program. The Windows Messenger Service is simply a notification service that is meant to allow system administrators to pop up a message box in the middle of your screen with an alert of some sort. Unfortunately, anyone can send such a message, and it's the source of many spam pop-ups.

To turn it off, right click on My Computer, and select Manage:

Expand Services and Applications by clicking the boxed plus sign to its left, and then click on Services:

Scroll down until you find a service named Messenger:

Right click on Messenger in the list, and click on Properties:

Click on the Stop button to stop the service, change the Startup Type to Disabled and click Apply. The result should look much like the above.

Other Services

What if you really, really want to turn off unnecessary services?

Well, to start with, prepare to start doing some research and some trial and error experimentation. As I said to begin with, "it depends". What can be turned off safely for one computer might cause problems for another, depending on how they are configured, and how they are used.

First, you can quickly see which services are running the Computer Management dialog above my making it wider, or simply scrolling the list to the left to display the Status column. "Started" means that the service is currently running. Scroll a little further left and you'll see a default description of that service.

One of the best resources for understanding what Windows Services are, and whether or not they're needed was a site by someone who called himself "Black Viper". Sadly his site disappeared suddenly, but thanks to the Internet Archive, a copy of his Service Configurations page still remains. Here you can see not only somewhat more detailed descriptions of the services, but also his recommendations at the time as to which services might be required. (That information is also mirrored at Major Geeks.)

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The process to then go through is to examine each service, determine if it might be optional for you, and if so, try turning it off. Caution: you're playing with fundamental Windows configuration here, so a) backup, b) be prepared to reboot into safe mode if needed to recover your system, c) make sure you have your recovery console and/or Windows XP CD available, "just in case", and d) backup.

But like I said at the beginning, in my opinion it's much safer and quicker so simply let sleeping services lay, and just add some RAM to your system if you feel so motivated.