The fact is, you don't need all of the services that Windows starts automatically when it boots. Disabling the non-essential services frees up memory and processor cycles for more important tasks. The trick is knowing which of Windows' automatic services you can do without: disabling the wrong service can render your system unusable. If you're careful, you can figure out which automatically enabled services your PC can do without.
Things would be so much simpler if I could just list which services to disable, but each Windows configuration is unique, so there's no way to predict which ones are required on your system. That's why I rely on sites such as Charles Sparks' BlackViper.com.
First, play it safe by setting a restore point
Start by backing up the Registry (the page also describes how to restore it). Next, open the Services applet: In XP, click Start>Run, type services.msc, and press Enter; in Vista, press the Windows key, type services.msc, and press Enter. (Avoid the temptation to access your services via Msconfig, aka the System Configuration utility.)
BlackViper.com's list of XP services shows the default settings with Service Pack 2 installed. Likewise, the site's Vista services list assumes that you've downloaded and installed all "important" updates for that OS.
You'll likely find more services on your system than are listed there, most of which were installed by software you or the PC's vendor added. You may also find services on the BlackViper.com list that aren't on your machine (especially if you use XP Home); some OEMs choose not to install some services. Work your way through the services, disabling those enabled by default that you deem unnecessary. You can play it safe by setting a service on Manual, which starts it only when Windows decides that your system needs it. Unfortunately, some services set to Manual won't start when they should, so you may need to reset these to Automatic.
To determine which other services a particular entry requires (and which other services require it), double-click its entry in the Services list to open its Properties dialog box, and click the Dependencies tab. Along with the suggestions on the BlackViper.com site, look for services relating to hardware you no longer use. Other candidates for disabling are Remote Registry, Themes (if you're happy with Windows' Classic appearance), and Windows Firewall (only if your system is protected by a third-party firewall). Note that changes you make here apply to all users on the system.
Tomorrow: A free utility that removes unwanted start-up apps from the System Configuration utility (Msconfig) once and for all.