A Windows service provides functionality to the OS and user accounts, regardless of whether anyone is logged on to a system. Windows XP comes with about four dozen services enabled by default.
A question many Windows administrators ask, therefore, is which services can I safely disable? What if I told you that for at least basic functionality such as Web surfing and application execution, Windows doesn't need any services? In fact, you can also do those things without system processes such as the interactive logon manager and the local security authority subsystem.
After some experimentation in which my colleague Dave Solomon carefully disabled services and processes to achieve a minimal Windows system, we were dumbfounded to discover that Windows was usable without all the core system processes. We then polled senior Windows experts such as the vice president of the Core Operating Systems Division, the technical lead of the Virtual PC team, and a lead Windows security architect to see whether they thought that Windows would function at all, much less if Microsoft Internet Explorer (IE) would work, without the support of services or the Winlogon and Lsass processes. The unanimous answer was no. Even after we showed them the results of our experimentation, they all thought that we'd staged some kind of trick. You can learn how we achieved a minimal Windows configuration and perhaps experiment with your own machine by following the detailed instructions in my blog posting at http://www.sysinternals.com/blog/2005/07/running-windows-with-no-services.html
So what are the real limitations of running with minimal services and processes? Some will become obvious during your exploration, but a major one is that you won't be able to log off (or shut down) because neither Lsass nor Winlogon is running. The bottom line is that this stripped-down Windows configuration isn't practical, but it makes a cool demonstration of just how little of Windows is required for basic functionality.