I work with WinXP Prof, and am connected to a local server with ~ 100 other computers, by an external USB modem on DSL.
Usually my computer worked with 8000/600 kbps speed, but lately it went down to ~2050/670 kbps.
How can I find if the fault is in my motherboard, the modem or the server?
Thanks

If the modem or server, the other users should be complaining too.

I have no contact with other users and the modem sits right here on my computer.
Is there any way to check what goes wrong?
Can some problem in mu USB outlets be found and how?

am connected to a local server with ~ 100 other computers, by an external USB modem on DSL.

I have no contact with other users and the modem sits right here on my computer.

I'm sorry, but I'm confused. I don't understand your network. What does the server and 100 users have to do with you then?

I have no way to ask other users about their connections.
My modem belongs to my PC an dI want to check it.

I ask again, what does the server and 100 users have to do with you? Do they go through your DSL modem? How do they connect? Is there a router somewhere?

my PC is connected to a LAN server through an external XAVI modem which sits on my computer and is connected to an USB port.
I want to know if the problem lies with the LAN server (and I cannot ask other users about their speed),
or with my modem, or with the USB outlets on my computer.
By the way, I checked at least 3 of the 6 USB outlets, and the result is the same.

I am sorry but I don't know the answer. Your configuration seems strange to me. I don't see why you need a DSL modem to connect to a server on your LAN. It seems to me you should be connecting directly (or through a router or switch) to the server.

Typically, a DSL modem is the "gateway device" that connects directly to the Internet access point provided by your ISP, and not through a local server.

Does your computer work fine otherwise?