For a registration purpose I am entering some data.. and again for other iterations the testcase going to fail why because...session cookies are still presentand entire test cases are going to fail .. what I have to do...clearcache is also not working...

Answer1:

We had much of the same kind of issue. The only way around this was to not accept cookies, if the application allowed for it or we killed the browser between tests and went and deleted the cookie repository directly.

Answer2:

You will have to write a small code that will clean the cache - Browser Tools menu->Internet Options, then Click on Delete Cookies and Delete Files. You can make this a part of your App state so that it run everytime before you start a test case.