Windows 7 - Weird computer identity theft by local machine

Last weekend I upgraded my company net from 100mg to 1 gig (new firewall/router; all new cabling). Small net: 4 machines/peer-to-peer. All machines are Windows 7 64 bit. We use my machine as a sort of server (not domain) for the accounting software (Quickbooks) as well as document files (all vendor invoices).

Let me make some defs so you can follow:

Workstation 1: my machine: hosts the accounting data base as well as all the scanned documents.
Workstation 2: one of the interior designers
Workstation 3: another one of the inter designers
workstation 4: retail store check out.

Routine plan: workstation 1 stays on all the time. Others shut off at end of day. In the morning, workstation 2 usually cranks up before others arrive.

Since upgrading the net, if we fail to launch workstation 4 AND connect it to the accounting database on workstation 1, workstation 3 takes on the identity of workstation 1 but doesn't host the accounting database causing workstation 4 to be unable to connect to the accounting database. (When you go to the network and click on workstation 1 you actually see the icons for workstation 2).

If I shut workstation 2 down (cold) then workstation 4 readily connects to the database on workstation 1. Once re-booted, workstation 2 happily goes about its duties.

Ideas????

Hi Masterscuba, welcome to sevenforums. Try setting up fixed ip settings in both your router(which probably has DHCP set) and the other machines. This could be caused by the DHCP server trying to "issue" ip addresses taht are already fixed in 1 machine(probably number 4)