ModuleID is a property of Task of type Guid.

This is how I solved the issue:

void PopulateModuleList(Guid selectedModule)
        {
            List<Module> modules = SecurityModuleViewModelBuilder.ConvertToModules(repository.GetModules(true));

            if (selectedModule != Guid.Empty)
                ViewBag.ModuleID = new SelectList(modules, "ID", "Name", selectedModule);
            else
                ViewBag.ModuleID = new SelectList(modules, "ID", "Name");
        }

        public ActionResult CreateTask(Guid? id)
        {
            Task task = new Task();

            if (id.HasValue && id.Value != Guid.Empty)
                PopulateModuleList(id.Value);
            else
                PopulateModuleList(Guid.Empty);

            return View(task);
        }

This code populates the dropdown, if an ID is passed it is set as selected and if not it just displays the list.