Many admins learned how to create reports by opening up the log files in ISA 2000 and using Excel features to organize the data in a meaningful way. Contrary to popular opinion, you can use Excel to generate a report using ISA 2004 with MSDE logging much easier than in ISA 2000 flat files.
Start by trimming out what you don't want to see, right in ISA.
In the monitoring tab create a query with the information you want to view.
Logging last 7 days
Action Allowed Connection
Rule SBS Internet Access Rule
Client Username Not Equal Annonymous
This will display in the monitoring viewer a list of packets going to websites. Press the Copy to Clipboard and then paste into Excel to start organizating the data into a report.