Is 100Mbit network for 24 gaming pc too slow?

Hi.
I got 24 computers on a lan + a server.
The network has 2 switches that are both 10/100Mbit capable.
All the computers have 100Mbit NICs in them and are all connected with blue Cat5e cables to the two switches.
The server is Pentium 3 700Mhz:D, 1Gb ram runing Windows Server 2003. connected straight to one of the switches through a long Cat5e cable.
But while playing, say, Counter-Strike 1.6 over Lan with 6v6, the lag is around 30-70, and when people join and don't have a certain map on their PC it takes them to download it for about 3 minutes.

I mean 100Mbits c'mon, this network should be fast enough, but it isn't and why is that? Help.

The whole probles is "The server is Pentium 3 700Mhz"

That thing can not feed the computers fast enough. Definitely time to upgrade that server!