How do you setup and small office network using vista OS

I have a small office with only three comupter two desktop and one laptop
all running on vista. we vpn into a sever to help with billing and appointment setting. This company server is all ways down making it difficult for us to accomplish out billing and appointment setting. Can we set up out own small office network and purchase a server to connect to this company server in order to stay connected to there server? We really need to use this software that's on there server because it does our billing and appointments. I not sure if we really need a server. But we do need a small office network. We also use AT&T SBC global for our internet connectivity that runs at 1.5 internet speed. Please some help.