Intel iMac crashes on network

At our school, I have just installed Leopard 10.5.6 on 26 Intel Macs (first intel model) along with a mac server to handle WGM. Users are authenticating using Active Directory on a Windows 2000 server. Since setting up, we have had random crashes on the client iMacs, mostly when users log off. Crash is either a blue screen freeze, a message to re-start or a partial screen blanking and freeze. A complete re-start is required.
Any ideas why this is happening? Never had this before the above network setup.