The most probable cause of any single computer being slow it that the Half/Full-Duplex setting is wrong on the computer. There is a protocol that should allow cards to auto-configure, but it isn't that reliable and manual settings are often more successful.
The "Rules"
If your computer connects to a Switch, try forcing the network card in the computer to Full-Duplex at the highest speed for the switch (probably 100mb/s)
If your computer connects to a Hub, try forcing the network card in the computer to Half-duplex at the maximum speed of the hub.
Do not mix Hubs and Switches in a network because you end up with a half-full duplex problem between them and, unless you have commercial class switches, can't resolve it.
The Reasons
Still asking "huh??" ...?
A switch has a number of ports that feed into buffers, and packets come in one port, get shifted to one other port, and sent back out. So a switch makes a dynamic connection between two ports based one where data is coming in and going to. It operates in Full-duplex, so it can receive data that comes in port 1 that is going out port 2 (as an example) at the same time a data comes in port three that is sent out port 1.
A hub uses a bus; all the computers use the same bus to send and receive and there is nothing in the hub to sort it out; when data comes into port 1, it goes out at the same time on all the ports, including port 1. This means the computer connected to hub must do two things it does not need to do on a switch (and these define Half-Duplex):
1. While a computer is talking to a hub, it must wait until the network (what is coming out of the hub) is quiet before talking to avoid collisions. Port 1's computer can't talk to port 2's computer while port 3's computer is also talking to port 1 using a hub.
2. While a computer is talking to a hub, it must ignore data on the network -- it shouldn't be listening to itself, after all.
When Bad Duplex happens to Good Computers
If a computer is in full-duplex on a hub, it will tend to talk over top of everyone else on the network, this causes a lot of collisions and everyone gets slow.
If a computer is half-duplex on a switch, it will tend to ignore other computers that talk while it is talking and/or assume that being interrupted by someone else talking means there was a collision and it should resend the data. Either way, the one computer will be slow.
Ted