MSConfig, similar to the application included in Win9x of the same name, allows the user to fine tune the applications that are launched at startup without forcing the user to delve deep into the registry. To disable some of the applications launched, load msconfig.exe from the run command line, and go to the Startup tab. From there, un-ticking the checkbox next to a startup item will stop it from launching. There are a few application that you will never want to disable (ctfmon comes to mind), but for the most part the best settings vary greatly from system to system.
As a good rule of thumb, though, it is unlikely that you will want to disable anything in the Windows directory (unless it's a third-party program that was incorrectly installed into the Windows directory), nor will you want to disable anything directly relating to your system hardware. The only exception to this is when you are dealing with software, which does not give you any added benefits (some OEM dealers load your system up with software you do not need). The nice part of msconfig is that it does not delete any of the settings, it simply disables them, and so you can go back and restart a startup application if you find that you need it. This optimization won't take effect until after a reboot.