I think you are not understanding what I said....and I know that I do not understand what you are saying....what is VMing?? I am definitely continuing to use the old computers, that is the point, how do you keep them on your active network but not have them connected to the outside world.
I understood what you were saying. Either one of those situations would allow you to continue to hookup to a network and use it, although how you would do it would be different.
VM is short for Virtual Machine. It is essentially running a computer within a computer. But instead of having a separate "tower" for XP or Win 7, everything is handle by software and your entire XP or Win 7 computer are contained within a single file (if your are trying to replace say 4 computers, they will all be within their own individual files). VMing is a way to use the older systems without having to maintain older hardware (which is going to get harder and harder to fix/replace).
If you want to use existing hardware and not VM, then I would suggest making sure that the server that you want to connect to with your legacy computers does not have internet access. If it has access anywhere to the outside network, the potential for your legacy equipment to get infected is greater. There is some malware that can attack files even through network shares.
Now, if you go the VM route, then you would want to get to network shares by using "shared folders". Essentially the network share is mounted to the host OS and that mount point is used as a "shared folder" that the guest OS (legacy OS) can see and use. You would have to scan all incoming files to make sure that it's not going to affect the legacy OSs (same thing that you would have to do if using older hardware with a server that's connected to the outside network), but the beauty of a VM is if something gets infected, it's easier to have a backup of one file and just delete the infected file and move a clean copy of the backup in it's place.