All-encompassing (or combined pieces) company email and Internet monitoring.

I am being asked by a number of my clients for either general reports on company Internet usage, or specific monitoring of either Internet sites visted, time spent, even number of emails sent.

I am looking for a solution I can use on site to track this type of activity.  Ideally, I would like to have a program, or suite of programs, I can put on a machine, put on the network (somewhere) and be able to generate reports.  I would like to be able to report back:
- these are the sites being visited
- this is when Internet is used the most
- these are the sites _this person_ is visiting
- this is how much email this person is sending/receiving

I have looked at Webtrends and few others, but would like some 'real world' feedback on what products have worked for the experts here.

I work exclusively with Windows machines, so no concerns with Mac or Linux, etc.

I would also like to avoid anything that I need to install directly on a desktop.  This condition would go as far as modifying the connection settings on IE.  I am not discounting solutions that require you to do that, but at a site with 100 pcs, I would rather not.

Any helpful feedback would be appreciated.
Also, all of these offices have a pretty comprehensive Acceptable Usage Policy, with warnings about privacy and monitoring.

Thanks,

TN