The following code has been tested on Windows 2003 SP2.  The calling user account must have 'Assign Primary Token' and 'Increase Quota' permissions to properly execute CreateProcessAsUser; see MS KB285879. 

Public Function RunProc(ByVal CMD As String, ByVal ARG As String) As String
        Dim er As Int16
        Dim exitCode As System.UInt32 = Convert.ToUInt32(123)
        
        Dim saThreadAttributes As SECURITY_ATTRIBUTES = New SECURITY_ATTRIBUTES
        saThreadAttributes.nLength = Marshal.SizeOf(saThreadAttributes)

        Dim impToken As System.IntPtr = IntPtr.Zero
        Dim priToken As System.IntPtr = IntPtr.Zero

        If LogonUser("DomainUser", "Domain", "Passwd", LogonType.LOGON32_LOGON_INTERACTIVE, _
               LogonProvider.LOGON32_PROVIDER_DEFAULT, impToken) Then
            If DuplicateTokenEx(impToken, &H2000000, Nothing, SecurityImpersonationLevel.SecurityDelegation, TOKEN_TYPE.TokenPrimary, priToken) Then

                Dim pi As PROCESS_INFORMATION = New PROCESS_INFORMATION
                Dim si As STARTUPINFO = New STARTUPINFO
   si.cb = Marshal.SizeOf(si)
                si.lpDesktop = IntPtr.Zero

                Dim saProcessAttributes As SECURITY_ATTRIBUTES = New SECURITY_ATTRIBUTES
                saProcessAttributes.nLength = Marshal.SizeOf(saProcessAttributes)

                If Not CreateProcessAsUser(priToken, CMD, CMD & " " & ARG, saProcessAttributes, _
                saThreadAttributes, False, 0, IntPtr.Zero, "c:\", si, pi) Then

                    er = Marshal.GetLastWin32Error
                    RunProc = ("err runas  " & er)

                Else
                    WaitForSingleObject(pi.hProcess, Infinite)
                    GetExitCodeProcess(pi.hProcess, exitCode)
                End If
                CloseHandle(priToken)
                CloseHandle(impToken)
                CloseHandle(pi.hProcess)
                CloseHandle(pi.hThread)
           End If
        End If
End Function