Sysinternals checks out how many system processes you really need to run Windows XP. But the bottom line is that such an extremely stripped-down Windows XP isn't really practical..
A Windows service provides functionality to the operating system and user accounts regardless of whether anyone is logged into a system. Windows XP comes with around four dozen services enabled by default, including ones that many people consider superfluous like Remote Registry, Alerter, and SSDP Discovery (Universal Plug and Play). A question many Windows administrators commonly have is therefore, which services can I safely disable? What if I told you that for at least basic functionality like Web surfing and application execution, Windows doesn’t need any services? In fact, you can also do those things without system processes like Winlogon.exe, the interactive logon manager, and Lsass, the local security authority subsystem.