Installed two new, identical PCs to a business network. Network eqipment is basically a Netgear ProSafe switch and router, with a Buffalo Terastation as NASdrive.
All the older PCs behave fine. The two new PCs occasionally (1 boot in 8 or so) hang after boot up. Everything appears to load, and theres nothing unusual in the event log when looking later, but the system freezes. Mouse can move, but thats it. Nothing. On reboot, its fine. Both PCs that freeze are ASUS P5945GC motherboards, C2D cpus, 4.0Gb Buffalo twinx memory.
Occasionally they freeze during use, but only when saving to the NAS drive.
The only error showing in the event log is:-

"Event ID: 1003"
"Your computer was not able to renew its address from the network (from the DHCP server) for the network card etc etc etc
The semaphore timeout period has expired. Your computer will continue to try and obtain an address from the network address (DHCP) server."

But they connect just fine.

I have tried replacing network adapters. They were running ESET Smart Security (as the rest of the PCs), but the problem still arises without it on as well.

Any clues?