Routing multiple branches into different subnets at Head Office

I have four different branches coming into the Head Office via leased line.

What type of "leased line"?  What type of hardware are you using to connect the sites (Cisco, Netgear, etc.)

I currently have the default gateway of all the routers as being a PC that has multiple NIC's and I route them to the different subnets via routing on the PC.

The PC is doing the routing instead of the routers?

Hi Thanks for the reply.  They are Cisco routers that are not under my control (Service Provider) So I had all of them configured with the PC as the default gateway.  That way I can add and change routes on the PC without having to bother with the SP.   So yes, the PC is doing the routing not the routers.

Can you post a topology diagram? I can't figure out how your network is setup.

Thanks for the reply.  The attatched graphic show a number of different companies, branches and sub branches all connected to the Head Office via cisco routers.  All the traffic on all the routers is directed to the single workstation.   Routes are then added to the workstation as necessary.   The servers are on three seperate subnets and there are three NICs on the workstation to route to each of the subnets.   Each companie/branch/subnet needs to access only certain servers on each of the sebnets.  

This is working correctly but my question is, is there a bettrer way  bearing in minbd that I do not want to chnage routes on the routers themselves.   Esecially as there are more connections in the future.   Hope I made myslef clear.  Many Thanks
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Thats just the sort of information I was looking for.   Could you recomend a cheap multilayer switch?   I checked out the Cisco 2960  switch but is stated as being a fixed confiuration switch which I gather wouldnt work.   Any other choices?    MAny Tha ks for your help

Why do you need a modular switch ?

Sorry, dont get that one.   Looking for anything that can achieve what we have been discussing above.  When you suggested the Cisco 2960,  I took "fixed configuration" to mean that it could not be configured as we would require.  That not right?   Again, looking for any device that can route the way we discussed.   Thanks for the reply

No. "Fixed Configuration" means that it's not a modular switch. Which means you can't add ports or change the types of ports. i.e. if you buy a 24-port UTP switch, you're stuck with a 24-port UTP switch. You can't add another 24-ports later.

I understand now.  Many Thanks.  So, back to looking for a cheap switch that will do the routing I described to replace my old PC.   From research I gathered that the latest versions of the Cisco 2960 can route ( if it has the right os ) is there any others you can suggest?

HP and Netgear are alternatives. Just make sure that it can route. You're looking for what's referred to as a "multilayer" or "layer 3" switch.

Thanks for all the help