I'm more used to doing this for groups in active directory but, you may have success doing it for few machines by going to Start / Run / gpedit ...
On the Redhat box, I find it much easier to use webmin for managing squid acl lists. We block everything then, add allowed sites to the allow-access url_regexp.
# Edit Proxy Group Policy in domain
\\Server1 / Master.msc / Someplace.com / Sales / Properties / Group Policy tab
Proxy Policy / Edit
User Configuration / Windows Settings / IE Maintenance
Connection / Connection Settings
Do not customize Connection Settings
Connection / Proxy Settings
[X] Enable proxy settings
Address of proxy Port
[X] Use the same proxy server for all addresses