virus spreading through networks?

Hi I am currently employed at this computer store

we do many computer repairs. Ive noticed that 90% of the time the computers are infected with viruses/spyware. Ive been researching how to control the viruses from spreading over our private network and infecting all the other computers in our business. I just cant seem to find a solution. I have tried googling allot and cant find anything. Can someone please tell me what i need to do to keep the viruses spreading to other computers in our network. I want the repair computers to be able to access the internet but not be able to access the other computers in the business. This is a basic knowledge of how our network is setup.

internet > linksys router > (3) sales rep computers
> switch > shop computers
> linksys router > repair computers

Thanks in advance