Ok, it would appear that even though Microsoft .NET Framework Version:2.0.50727.4234; might appear on screen, version asp.net 3.5 could still be installed. I've checked out a few sources online, including this "asp.net version madness" :
http://blogs.msdn.com/b/jamesche/archive/2007/09/25/asp-net-version-madness.aspx
So I don't believe the error is now caused by running a lower version.
As for the error I'm getting, I think I need to add this to my web.config:
<add assembly="System.Web.Routing, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>