Hi Favek,
If you are using ISA Server 2006 you can go to ISA Management Console, then on the left pane go to Monitoring, then go to Logging Tab, then edit filter, then add the "Client IP", on condition - equals, then put your IP Address eg:
10.10.88.40, then save, then start logging, then at your desktop PC try to browse
http://www.youtube.com, but you need to go to your IE - internet options, Connection Tab, then LAN Settings, then at Proxy Server - put or place your proxy server and port - this will be base on your ISA Server setup or
configuration, Then go to Monitoring, then Logging Tab you will see all the activity and it will show all related website links which included when you browse the
http://www.youtube.com, then get all website then add this to your created firewall policy which is blocking.
Hope this will help and try to research with
http://www.isaserver.org you will get there more of information about ISA Server and TMG Server.
Thanks,