In our network there is an ISA (2000) webproxy. It forwards requests to various IIS servers further down. Each request results in a log entry on the ISA server and another log entry on the IIS server the request is forwarded to. How would you match up these log entries ?
The only way you can match them is by corresponding URL, date and time, and in the IIS log the client IP will be that of your ISA server. The logs are independent of each other so there is no concept of automatic correlation, you'll have to analyze it.
There might be tools by now (it's been a little while since I've done ISA administration) that will help you automate that analysis but I will admit I don't know of any.