Currently I have the source of System.Web.Mvc assembly. Building is fine. But at runtime it throws,
Could not load file or assembly 'System.Web.Mvc' or one of its dependencies.
Strong name signature could not be verified. The assembly may have been tampered
with, or it was delay signed but not fully signed with the correct private key.
(Exception from HRESULT: 0x80131045)
It seems that you are trying to debug the ASP.NET MVC source code and have built your own version of the System.Web.Mvc assembly. The problem with this approach is that you cannot sign it with the official keys. This means that any third party component that you might be using and which depends on System.Web.Mvc should also be recompiled against your own version. Take for example Razor. It also depends on System.Web.Mvc. Did you recompile that as well?
Personally I find it extremely difficult to be building your own version of System.Web.Mvc. In practice I debug the source code by using the publicly available PDB symbols. So I would advice you NOT to be compiling your own version but work with the official one. Take a look at this post: http://stackoverflow.com/a/13610108/29407