your connection speed is way to slow for that many users. you should be useing a commercial ISP package for anything over 10 users (at least 6Mbps (download) / 800Kbps (upload) speeds)

I have to agree with ian_422. Even with 6Mbps connection with that many users, if you have someone streaming audio or video it's going to bog down your connection. I would recommend a T1.

thank you for your help.
what is T1?

I have been a sysadmin at a school that had 128KB ISDN line for over 400 workstations and everything worked

look intop the network usage, sniff the traffic on the gateway - somebody is probably using P2P programs

T1 is a internet connection that the bandwidth is guaranteed. Where as with cable you are sharing bandwidth with your neighbors and DSL depends on your distance from the central HUB. T1 usually has 24/7 support so if the connection is lost in the middle of the night they will send someone out immediately. For any business of more then 10-15 employees I would recommend a T1.

First thing to do is to get the knowledge on how is used your Internet connection.
I mean to get the traceability of all network echanges.
Secondly a good thing would be to put some thresholds alerting you in case there are some abuses by some workstations.
For those 2 points, there is a very interesting free tool named NER(Network Exchanges
Reporter).
URL is: http://www.showtrail.com/
I hope it could help.

great, another spammer