Seems like the problem is with iusr account.
To confirm this,
I would like you to put in your current logged in account in IIS
in place of IUSR_SERVERName
and password
try accessing the website....if it works fine then we sorted out the problem.
we shall concentrate on IUSR
In ad just check if IUSR is member of users...sometimes it will be member of guests and guest account would be disabled.
let me know the results..