I've been a software engineer in .NET for the past 3.5 years, and did my own integration and unit testing, but nothing too extensive. Now I'm being interviewed for an entry level QA job, and I wanted to know what concepts I should become familiar with.
Alright since now you are moving to QA positions here are some points to ponder:
1. Communication Skills: You must have good communication skills as you will be dealing with Developers, Managers and clients at same period of time.
2. Patience: Sometimes executing test cases again and again (Regression testing) can be a tidy job so you must be patient enough to do it without losing focus.
3. Testing Tools: There are lots of testing tools out there like HP QC, Load Runner, Selenium, QTP etc you should get familiar with them as they are tester's tool. (go to softwaretestignclub.com or guru99.com they are good sites).
4.Design Methodologies : As you were working with .Net for a long time i assume that you will be familiar with Agile, TDD etc concepts.
4. Here you can find a very good white paper listing what you should need to be a successful QA.
You might think this is a long list but i am sure as you will enter in an organization and start working you will learn the things with time. Good Luck!
Good luck with your interview. It sounds like you need a quick overview of general testing concepts - I'd recommend taking a look here:
If you're short of time before the interview, I'd recommend you at least look at lecture two, Strategy - "This considers why testers test, what they are trying to learn, and how they can organize their work to achieve their mission." This will give you a context that will help you to make the change of mindset you'll need to be successful both in the new role, but more importantly before that in convincing the interviewer that you understand there are new skills you'll need to learn, and you're keen to learn them!
I'd also recommend strongly that you try to find the time for lecture 3 on oracles (a really key concept for new testers to grasp), and lecture 5, which explains why complete testing is unattainable.
The other lectures in the Foundations section are well worth looking at, but I think with your programming background, those three are the ones that will fill in the most missing background for you quickly. I'd recommend you explore the rest of the course as and when you have time. It's well worth it.
I also really liked Alan's answer to this question, which explain what he's looking for as an interviewer: What are good interview questions for a Software Tester with focus on automation?