How come my networked computers download from the server and use internet, but they can't upload to server?

I work in a small company and we have a bunch of computers networked together. All of the computers can download from the server very fast and surf the web without any trouble. But when I try to upload to the server from any of the computers it either takes for ever or crashes completely. What's the deal? This is the case for all of the switches, so moving them to different ports on the switch doesn't help.How come my networked computers download from the server and use internet, but they can't upload to server?
Depending on the type of topology you are using you may be getting major collisions on the upload. Have you considered using another type of topology? I am not sure as to how many pc's you have on the network. I would be able to suggest a type.