How to Block IM

I would like to know how i can block IM activitys from other PC’s on my LAN.
All Pc’s are connected trough my PC with a LAN and i have internet connection trough a Cable connection. Now i would like to see and to block any activities from any of the other PC’s.
I am running Winxp Pro Sp2 on my own and SP1 on the rest of the PC’s.
Any help would be appriciated.
thanks.