How do I write test automation tools like QTP and winrunner using .net?

In fact, WatiN is a good place to start. It's not really a unit testing system. It may look like one at first, but it is used to write a functional test. It's entirely up to you if you run them as unit tests. I wrote the same test using WatiN in two ways (functional test that simulated user actions on a web page):

1) Writing a script in Powershell when run from the command line just like any other PS script. It was fun, although you have to write a lot of code for reporting, exception handling, and more.
2) Writing a unit test to MSVS in C # using the C # unit test from the MSVS project type. It was fun because you just ran it in MSVS like a unit test, but you have support for an environment to write code, generate reports, work, etc.

So, if you want to start with something to look at WatiN, especially since WatiN has a special recorder that records actions and outputs the code in C #. By looking at the internals of the library and tool, you get started.

Only one thing is just the web. The desktop is completely different. With web, you have a hook in IE, you can query the html document for objects, check the browser state, and so on. With a desktop, this can be tricky. You need to connect to the app, possibly through the Win32hooks mentioned. Maybe try with Microsoft Scripting Host.