Network architecture question.

I have a bit of a dilemma on my hands right now.

I'm in the process of upgrading from a SBS2003 box to SBS2008 in our office. I currently have our network segmented because we run a computer repair shop and many customer computers come in with viruses and malware that i want to keep segregated from our server and office PC's.

Currently the network looks like this:

Modem -> Router -> SBS external NIC -> SBS internal NIC -> dumb Switch -> Office PC's
\-> dumb Switch -> Customer PC's.

So the router manages DHCP for the customer PC's while the SBS box (external nic) is assigned a static IP outside of the DHCP range of the router. The SBS box manages DHCP on the internal NIC side for our office PC's.

SBS 2008 doesn't support this configuration...

So in your opinions, what is the best solution to allow my SBS2008 box to enter the network and still be able to keep my office PC's segregated from the customer PC's?

I'd also like to avoid getting into a 2 router scenario if possible.