Measuring network traffic

I want to measure the network traffic to see where our client/server application is getting bottlenecked. What kind of apps are there to view this? The two computers I will be looking at are Windows XP and Server 2003.

Thanks.

So do you want to make your own network monitor or use one that someone else has already made?

For now someone elses. In the future I would like to build an app to do this. I found a website - portablefreeware.com that helped me out. Do you have any other suggestions?

Thanks

Well it depends exactly where/what you want to monitor...
I've used a program called WireShark a fair bit previously to inspect packets being sent accross the network from a PC but if you just want to see how much data is being passed in and out of a PC/server then I would of thought you could get quite a few just by googling "free network bandwidth monitor" or something like that