John Hammer1 is partially right. There are three locations for startup items
</System/Library/StartupItems>
</Library/StartupItems>
<~/Library/StartupItems>
(In theory, the first should be reserved exclusively for items bundled with Mac OS X.)
However, startup items have been deprecated since Tiger. Apple advises that only software which requires Panther compatibility should use them. The correct mechanism in Tiger and later is launchd. Therefore, if you don't find anything in the StartupItems folders, what you need to look for is a .plist file corresponding to your programme, which could be in
<~/Library/LaunchAgents>
</Library/LaunchAgents>
</Library/LaunchDaemons>
</System/Library/LaunchDaemons>
</System/Library/LaunchAgents>
(See "Creating Launch Daemons and Agents" in "Daemons and Services Programming Guide" in Apple's Developer library.)