ProxyNow! 5 is provided as a VirtualBox, Virtual PC, Vmware or Vsphere images.
That means it can run on any of your Windows OS that supports the above images or your Linux Servers.
Here are the basic requirement.
Presently, Windows (XP and later), many Linux distributions, Mac OS X, Solaris and OpenSolaris are supported as the host operating system.
Reasonably powerful x86 hardware. Any recent Intel or AMD processor should do.
ProxyNow! needs at least 1GB of RAM. On top of that you need to factor in the RAM needed for your host operating system. For example if you are using Windows XP, it requires at least 1GB of RAM to run smoothly. Plus another 1GB needed by ProxyNow!, that will be a total of 2GB required.
Minimum of 4GB of free hard disk space. But it is advisable to have more than 5GB to cater for log files. 10GB will be nice.
At least one network card to connect to the network..