Testing Network Performance

Hi everybody,
I'm looking for a good advice to measure the goodness of a remote
network.
The problem is that I can't access this network not directly neither
remotely and there are no many information regarding the architecture.
All I know (and I'm not sure about all the details) is that we have to
monitor a software running on a couple of computers with XP Pro
installed. These PCs are part of a EPON (Ethernet Passive Optical
Network) whose details have not been provided. The architecture
resulting is a star network.

As someone told me, performance in that network are far from being
satisfying and overall speed seems to change many times during a week.

I guess the problem is with the different switches (of course no
details again) to which each PC is connected, that seem to be shared
with other customers' devices (no one could tell me how many and of
what kind)

So basically, something must be done. Unfortunately, the only thing
that is feasible is to send a colleague of mine (who is absolutely not
a network guru) with a laptop with XP to physically connect to that
network and get as many information as he can.

I need to tell him to perform some tests to make me understand the
underlying architecture and how performance may vary during a medium
period (let's say a week).

I'd like to use only freeware software, so I thought to use Look@Lan
to find out other devices on the same LAN, but I also need some free
tools that could run during a week to retrieve useful information
about how often the network's quality is good and, when speed is low,
what devices is causing the lack of performance.

Please remember that this colleague is not really brilliant with
networking, so I don't think he could manage software like Wireshark
or TcpDump.

Any advice would be appreciate.