The Service startup mode can be changed in C# using System.Management namespace. a sample method is given below.

/// <summary>
/// This routine updates the start mode of the provided service.
/// Add Reference to System.Management .net Assembly
/// </summary>
/// <param name="serviceName">Name of the service to be updated</param>
/// <param name="startMode">Manual or Automatic. This parameter could probably use
/// an enum.</param>
/// <param name="errorMsg">If applicable, error message assoicated with exception</param>
/// <returns>Success or failure. False is returned if service is not found.</returns>
public bool ServiceStartModeUpdate(string serviceName, string startMode, out string errorMsg)
{
uint success = 1;
errorMsg = string.Empty;

        string filter =
            String.Format("SELECT \* FROM Win32\_Service WHERE Name = '{0}'", serviceName);

        System.Management.ManagementObjectSearcher query = new ManagementObjectSearcher(filter);

        // No match = failed condition
        if (query == null) return false;

        try
        {
            ManagementObjectCollection services = query.Get();

            foreach (ManagementObject service in services)
            {
                ManagementBaseObject inParams =
                    service.GetMethodParameters("ChangeStartMode");
                inParams\["startmode"\] = startMode;

                ManagementBaseObject outParams =
                    service.InvokeMethod("ChangeStartMode", inParams, null);
                success = Convert.ToUInt16(outParams.Properties\["ReturnValue"\].Value);
                //custom method
                errorMsg = GetErrorMsg(success) + "\\n Try \\"Run as Administrator\\" ";
            }
        }
        catch (Exception ex)
        {
            errorMsg = ex.Message;
            throw;
        }

        return (success == 0);
    }