The Microsoft System Configuration utility is a fantastic tool to monitor which programs your system is running. You can see what services are loaded at startup and tell how much CPU those programs are demanding. If you go in and tinker with the default setup, it's pretty easy to disable the wrong service and cause your PC to malfunction. With so many services and programs listed, remembering which ones you edited can be daunting. At this point, the best thing to do is reset the configuration to a default state.
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Click the Windows "Start" button, type "msconfig" in the search bar and press "Enter" (XP users will need to click "Run," then type "msconfig" in the Run search bar and press "Enter"). A new Window should appear.
Look under the "General" tab and select "Normal Startup" from the option list. Click "OK" to save the changes.
Restart the computer so that Windows loads up all the default programs and services. After the changes, you can still go back and manually disable or edit the services.