There are various protection programs that allow you to intercept software trying to add itself to system startup but the one I like most is also the smallest and simplest: StartupMonitor.
It just runs in the background, not even a tasktray icon, and when a program tries to add itself to the startup, a very basic pop-up window appears telling you what the program is; you then decide whether to allow it or deny it with a single click (which also closes the window). As easy as it gets, and in the months that I've been using it, it's helped me prevent numerous programs from being unnecessarily run automatically.
No worries. I downloaded it anyway and it seems to work fine with Vista 64 bit which is more than can be said for some programs. It has recognised some changes as I was making them. Nice find!