You can try this..... (I think it will work)...

On the XP workstation that is set up as a share.... open a command prompt window with the Administrator account (or account with Admin rights) and enter the following command....

net config server /autodisconnect:number

Where number is the number of minutes you wish to have idle users disconnected.

Give it a shot.

There is a registry hack... but that is probably the easiest method.

Maybe you could consider setting up a Samba server using some version of Linux or Unix. It's free and you can have shitloads of people logged in at the same time. Samba is highly configurable to most needs.