Experiencing the networking lagging and slowdown

[xeroleacy]

Hi.

I am writing here to seek the helps on that matter of our networking experiencing slow down / lagging.

I am using the window 2003 server, for file server. All the pc ( about 20 pc ) accessing to file server for open file and editing and save there. I am using the switch(100mbps) to connect among server and pcs, assuming all the ethernet operate at 100mbps.

First day, seem so good, smooth enough. then after 4 month later, we had experience the networking slow down and transfer downtime.

For etc, when i open one file ( excel file ) consist of 400mb, it took long time to open it, about 15min. ( first day open it, took about 2min. )

And same thing for transfer file(2gb) to server, took about 7 ~ 15min for first time, and now it took longer than that, ( 30 ~ 40min )

Why is that so ? Is there a problem with networking or i feel that i am missing
the thing that i am suppose to do?

Please help.

Thanks and cheers