ISA 2006 IS BLOCKING MICROSOFT SITES AND ALL THE ANTIVIRUS SITES

hi,
I have a small problem in my isa 2006 ,i have a member server in my network on which i have installed isa 2006 for internet proxy , well all my clients can open most of the sites through isa but they cant open all the microsoft sites , most importantly the microsoft update sites and antivirus sites like mccafe , norton and symantec etc , neither the isa 2006 can open these sites from its own machine ,is there anybody who can help me in this stupid problem plz , it will be greatly appreciated thanks