It's only happening for one user where he keeps getting authentication pop up. After 4/5 attempts he can go to the document library page. Then when he clicks on a word file (where version and checkout is enabled) he gets endless pop up for credential.
No else having this trouble. I took following actions but problem still exists.
IE: Tools – Internet Options - Advanced tab – confirm Enable Integrated Windows Authentication is selected.
Not sure if i need to go through the steps in this link because about 2000 people are not seeing this but 1 employee. http://paulliebrand.com/2008/04/02/sharepoint-consistently-prompting-for-user-credentials/