The following code set the appropriate registry keys to login automatically and run a program once on the next login. This code runs on Server 2003, Windows XP and Vista.

Set objShell = WScript.CreateObject("WScript.Shell")

strUsername = "Admin"
strPassword = "Password123"

strRunOnce = "HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\RunOnce\joinDomain"
objShell.RegWrite strRunOnce, "cmd.exe /c cscript c:\install\myscript.vbs", "REG_SZ"

strAutoLogon = "HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\winlogon"
objShell.RegWrite strAutoLogon & "\AutoAdminLogon", "1", "REG_DWORD"
objShell.RegWrite strAutoLogon & "\DefaultUserName", strUsername, "REG_SZ"
objShell.RegWrite strAutoLogon & "\DefaultPassword", strPassword, "REG_SZ"