In short, you don't. You can also try out my "safe" configuration if you are scared about breaking something. Believe me when I say that every system is different and your requirements may very from mine. What I can tell you is that you can view my Services 411 Page for more information on each service and decide for yourself if it is required or not... Have I tested these? Oh yes. View the screenshot below. This is on a system that is on 24/7 but not a "daily user." I will have a screen shot of a computer that is used for everyday tasks and has been operational (even through various gaming) for 50 days.
Note: The system "idle" process is so low due to the computer running SETI normally through out the day and night for 48 days.
"Have you tweaked your OS lately?"
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Windows Service Configurations!
Includes explanations of each service and advice on which services you can safely disable.