You can also use Spybot S&D to edit your startup items, along with your registry.
Start>Run>regedit>HKEY_LOCAL_MACHINE>SOFTWARE>Microsoft>Windows>CurrentVersion>Run
It may seem long but it only takes like 15 seconds once you memorize the process. All you have to do then is figure out the name of the process and delete the entry (in this case, steam.exe?).
This can also be used to add programs you want to startup with windows. To do this,
Start>Run>regedit>HKEY_LOCAL_MACHINE>SOFTWARE>Microsoft>Windows>CurrentVersion>Run
It is the same location as before, except this time right click inside the white area next to the other values and select "New>String Value." Then type in the location of the program (.exe) either in quotes or not in quotes, I usually quote it, because I know that works, I haven't tried it without quotes yet. Voila! You can start up anything you want as long as it's an exe...