How can I disable Steam from starting up with Windows without logging in to Steam?

If Steam is running

In Steam, right-click on the icon in the the taskbar and select Settings, or from the full client, in the menu bar, select Steam, then Settings.

Click on the Interface tab, and in the middle of the window, clear the check box that reads Run Steam when my computer starts.

If Steam is not running

Run msconfig.exe. Under the Startup tab, look for the Steam entry and clear its check box. This will clear the corresponding setting in Steam as well. Even if Steam is restarted this will not be re-enabled without specifying in the application to do so.

If you want to do this without starting Steam and logging in, you can modify the setting in the Windows registry. The autostart checkbox is directly tied to a registry key — simply deleting it is all the Steam client does when said checkbox is unchecked.

Navigate to HKCU\Software\Microsoft\Windows\CurrentVersion\Run and delete the Steam key. Or as a .reg file:

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run]
"Steam"=-

1. Go to start menu
2. In the left hand corner search, type msconfig and hit enter
3. Go to the Start up tab
4. Untick tick Steam in list and reboot.