For any interview your basic concepts about .NET should be clear. initially they will ask you basic questions and then once you are able to answer the basic questions then they will ask you tough questions.
Mostly they will ask you about your previous projects and the current project, What technology you have used in your projects.
1.Net Framework : Garbage collection, components of .NET framework, GAC, class library, assemblies and types of assemblies, dll hell problem
2: C# & oops: Inheritence, abstraction, interface, difference between interface and abstraction, encapsulation, multiple inheritence, Design patterns
3: Asp.Net : Session management, state management, caching, web deployment methods, web page life cycle, drawbacks of state management techniques, postback, ADO.NET is very important
4: Sql : joins (queries), self join, stored procedures, functions and their differences, indexes, improving query performance
5: Ajax: Why ajax, what is ajax
6: WCF: difference between web services and wcf, ABC of WCF(address, binding, contract), deployment modes supported like self-hosting, IIS, WAS(Windows Activation service), protocols supported, supported bindings, SOAP, WSDL HTTP , SSL certificates and their deployment.
These are just few as I remember but please search online for 3+ years interview questions.
you have to sound confident and prepare the projects you have worked on previously
Microsoft Certified Technology Specialist in .Net