Client versions of Windows (Vista/7/8) have a maximum limit of 20 simultaneous network connections.
The cheapest version of server is Server 2008 R2 Standard.
If you purchase straight from the MSFT store,
Server 2008 R2 Standard with 5 Client Access Licenses (CALs) costs $1029.00.
If you use other vendors such as CDW, you may be able to get it cheaper.
Your costs will start to sore when you account for additional client access licenses.
If all you really need is just a simple file server for storage and such and you want the absolutely lowest procurement cost, use CentOS 6.2 (
http://www.centos.org/) or Ubuntu (
http://www.ubuntu.com/)
Linux.
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