IIS 7.5, Authentication Error 401

Setup
Windows Server 2008 R2
IIS 7.5
Active Directory

problem
setting up the authentication on a restricted site, i need 1 of my sites to only be visible to the two groups hgaLogon and hgaMedical, then 1 folder of the site to only be avalible to hgaMedical. how do i do this ?

i have tried different things but cannot get it right, i need an answer quick, preferable a step by step easy to follow guide thanks.

p.s. all groups in AD ...