When you say "download a 4.5 file in less than 5 minutes" what do you mean? Doesn't make much sense in its present form ;) You certainly couldn't download 4.5 Gb in 5 minutes.
I calculated the bit rate for that amount of data over your network based on the figures you gave. It works out at 15 Mbits per second. (4.5 Gb of data = 36 000 000 000 bits).
Given that your network cards can achieve 100 Mbits per second then that seems quite slow at first glance.
However, there are some other factors to take into consideration.
Hubs / Routers - Both computers are connected via another device whether that's a router or hub? If so the speed those devices are capable of is important. If you could give the make / model of the Hub we could eliminate that.
Slowest Device - In a network using Hubs the speed of the network is dependant on the slowest device attached to the hub. So if your router only runs at 10 Mbit and is connected to a hub then so does everything else.
Number of Files - Single continuous files take the shortest amount of time to copy. Hundreds of small files increase the copy time substantially.
Other Traffic - In any situation where a hub is involved only 1 device on the network is allowed to chat at any one time. If there is a lot of other traffic present then the speed of the network will quickly drop.
