In Windows Vista from the start menu search bar, type System Configuration, and click on the link that comes up. You'll be required to provide privilege elevation unless you've disabled UAC.
The Startup tab enumerates all the applications that are set to run as startup, regardless of whether they are set in the registry (either per-machine or per-user), the user profile, or simply the startup folder in the Programs entry of the start menu. You can disable any or all of them; they'll still be listed there so that you can switch them on again if you need to.
Nite: This tool also allows you to do some other pretty useful things, like editing your boot configuration database (no need to master arcane bcdedit commands any longer) or accessing a variety of other system tools and utilities. It's the power user's configuration tool of choice!