I remember back in Windows XP days that I read about disabling Telephony service (supposedly not needed if you are not using a dial-up internet connection) and cryptography (or similar) service (supposedly not needed if you don't encrypt your hard disk). Of course I didn't want to risk some incompatibility so I didn't do that. Even if I did the performance improvement would be negligible, I don't think these services use too much RAM or CPU time. One simpler and safer way to speedup a system is to disable all visual effects (transparency, fades, animations), all useless startup applications (browsers, update reminders and the like) and some non-Microsoft services. The latter are require more experience to figure out if you need them or not, so if unsure don't touch them. Of course all these imply that you have first updated all your drivers. I prefer to download the graphics driver directly from manufacturer (nVidia, Intel, AMD) to get the latest version and the full package including the Control Panel to tweak performance and appearance. Windows Update frequently installs an older version without any extras (core functionality, not Control Panel) and as you might know the default settings are not ideal.
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