Acces Denied for some accounts to root site but not to sites that inheret permissions from root

I´ve a SharePoint Foundation (2010) installation. No claims authentication, only Windows. Some accounts get access denied on the root site, but can access subsites that inherets permission from root site. I´ve tried to delete on of the problem accounts from sitecollection and add it back, a couple of times. Tried to give permission thru different groups and direct, nothing helps. New accounts have no problem to access the site. Is there a way to really start all over with an account?

Lena