Startup programs can also be configured in the registry:
HKCU\Software\Microsoft\Windows\CurrentVersion\RunHKCU\Software\Microsoft\Windows\CurrentVersion\RunOnceHKLM\Software\Microsoft\Windows\CurrentVersion\RunHKLM\Software\Microsoft\Windows\CurrentVersion\RunOnce
HKCU applies to the current user, while HLKM applies to everyone.
Entries under Run will run every time the user logs on, while those under RunOnce will only be executed a single time.
Create a REG_EXPAND_SZ registry entry under the appropriate key that points to the appropriate payload.