Sharing :: IP Conflict Causing Network To Crash

I am dealing with three office networks, all three have the same conflict. The error messages I get all say there are two devices assigning ip addresses. As soon as I get the computers communicating with each other, two seem to crash overnight, I suspect due to the conflict. Here is the rundown on the networks. All are windows networks - the problem I am having is file sharing mostly and printing.

Network A: server OS is XP Pro - SP3, four workstations, XP Pro-SP3, Windstream DSL and Netgear Router. I have already bridged the router on this one, and it did not correct the conflict.

Network B: Server OS - Windows7 SP1, three workstations all XP Pro SP3, ISP is Clear (formerly clearwire). Router is the old wired Linksys befsr41 v4.

Network C: server OS - XP Pro SP2, ISP - Clear, three workstations - two are XP ProSP3 one is XP Pro SP2. I have had less trouble with this network, but I suspect the conflict exists here as well.

I have tried to get a static IP address from Windstream where I have had my worst problems, but cannot seem to get one. I suspect we have the least expensive service option possible, and are possibly using a service designed for home use as opposed to business, as someone told me recently that getting a static IP address can be expensive. Is this true? Currently all networks have DHCP enabled, and the routers are set up to use DHCP. My thoughts were to disable DHCP and assign ip addresses manually.