Repeatedly Prompted to Log In

I just upgraded an old linux install to a version 6 windows install. Everything is work fine, but every now and then the users will be continually prompted to re-enter their credentials. The prompt is a standard web browser login prompt not the movable type login page or overlay. Anonymous access is on for the movable type directories. We only intermittently get his problem. I can clear the browser cache and cookies to and re login to solve the problem, but it keeps coming back. Would someone point me in a direction to solve this? Thanks.