You can disable services at start-up by using the system configuration utility, similar to the way that you halt programs from running at start-up, except that you use the Services tab instead of the Startup tab. But the system configuration utility doesn't necessarily list every service that launches on start-up. A bigger problem is that disabling services is more of shot in the dark than disabling programs. When you disable a program, you can get a sense of what the program does. But when you disable a service through the system configuration utility, there's often no way to know what it does.

• A better way of disabling services at start-up is via the Services computer-management console. Run it by typing services.msc at the command prompt. The Services computer-management console includes a description of all services so that you can know ahead of time whether a particular service is one you want to turn off. It also lets you pause the service so that you can test your machine and see whether that service is needed.
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  After you run the console, click the Extended tab. This view will show you a description of each service in the left pane when you highlight the service. The Startup Type column shows you which services launch on start-up--any with Automatic in that field. Click that column to sort together all the services that automatically launch on start-up. Then highlight each of those services and read the descriptions.
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  When you find a service you want to disable, right-click it and choose Properties. In the Properties dialog box that appears, choose Manual from the Startup Type drop-down list. The service won't start automatically from now on, but you can start it manually via the console. If you want the service disabled so that it can't be run, choose Disabled. To test the results, turn off any services that you don't want to run by clicking Stop The Service in the left pane, or by right-clicking the service and choosing Stop.

Here is a list of some common services you might want to stop from running at start-up. 

• Portable Media Serial Number - Retrieves the serial number of a portable music player attached to your PC.
•  Task Scheduler - Schedules unattended tasks to be run. If you don't schedule any unattended tasks, turn it off
• Uninterruptible Power Supply -Manages an Uninterruptible Power Supply (UPS) connected to your PC.
• Automatic Updates -Automatically checks for Windows updates . (You can check manually by going to http://windowsupdate.microsoft.com/.)
• Telnet (service available on XP Pro only - Allows a remote user to log in to your computer and run programs. (This will not be found on all versions of XP Pro.)
• Wireless Zero Configuration Service - Automatically configures a Wi-Fi (802.11) network card. Disable this only if you're not using a Wi-Fi network card.