DHCP Issues in Windows 2003

Hi,

I have a single Windows 2003 server which is configured as a DC and
provides DHCP services on the local LAN. I've found that, recently,
none of the PC's on the LAN are able to renew / obtain an IP Address
form the DHCP server.

I've checked the server and it is authorised in AD and the bindings are
correct. I've removed the DHCP service and reinstalled it. When I did
this I copied everything from %systemroot%\system32\dhcp to another
location and then deleted it.

I've checked the event logs and the DHCP.log files and can't find any
error messages. I've also checked the network cabling and replaced the
network switch.

The server does not have any firewall configured/installed and I tried
disabling the firewall on a couple of PC's in case there was an issue
there. As a last ditch attempt, I have reapplied SP1 but it's still
not working.

As a temporary solution, I've configured the router as a DHCP server
which is working but the clients can only talk to the server via its IP
address, even though the dhcp scope has the servers IP address for DNS
and the clients have this in their configuration.

Does anyone have any ideas? I'm completely stuck!