Oh, if I had money for every time people came to me saying "the network's slow" I'd be a rich man!
The very first thing to do is to ask questions. Who says it's slow? And who isn't seeing slow responses? All applications or just some? All servers or just some? Quite often this sort of exercise, coupled with a bit of logic, will tell you where the problem is.
Secondly, just because they say it's slow don't assume that it's the network. It might be the server or the user's PC
