You must be missing the System.Web.Routing assembly. Open the references folder and see if there is a warning triangle icon on that particular assembly.
Digging around I see that it was bundled with the .net 3.5 service pack 1 but I would also think it would be included in newer versions of .net. If you are running 3.5, install the service pack.
UPDATE: MORE INFO
In .net 4 it was moved into the System.Web assembly so you would definitely have it if you are running that version. It makes me wonder what he version of the web forms app is [check the web.config] and what you currently have installed on your machine.
You can see what assembly an object is in by searching MSDN. You can use the "previous" versions dropdown to see how it has changed over time or whether it was even available in previous versions.
http://msdn.microsoft.com/en-us/library/system.web.routing.urlroutingmodule(v=vs.100).aspx