Yes you can use ISA server on the inside network to filter http sites/monitor and generate http usage reports etc.
On your pix you'll need the following:
access-list inside permit tcp host any eq www
access-list inside deny tcp any any eq www
access-list inside permit ip any any
access-group inside in interface inside
Save with write mem and also issue clear xlate.
You'll need to point your internal users IE to your ISA server IP and the port number (8080 or 80)
I'm not a Microsoft guy so you'll need check up on ISA web proxy server setup etc, you can filter www sites on the ISA server.
Hope this helps and let me know how you get on.
Jay