Network share vanishes

I've been having problems with a network share recently. I have all my media stored on one Vista PC, but I tend to download it on my primary pc, also running vista. I've been using XBMC to watch network shared videos, but it loses connection completely if I connect to it from the primary PC to copy files over.
I know its not an XBMC issue, because the problem only persists until the server PC is restarted, and then it works fine until I next network into it. I've already disabled the firewall and AV but this hasn't changed it. Is there anything else which could be causing this issue, perhaps a userlimit somewhere, and if so how can I change it?

Thanks
Arkane