As Chopper3 writes, more expensive UPS' can have an option to attach networked shutdown software (for an additional cost).
For a free-of-charge implementation of networked power management you want APC UPS Daemon, apcupsd. Basically you install a manager instance of apcupsd on the PC that has the USB signalling cable attached, and install apcupsd clients on other PCs attached to the UPS power. The apcupsd instances use the LAN to communicate.
Apcupsd comes in both Unix and Windows versions, and the Windows versions have existed for quite some time, so I'd expect them to be stable enough. I don't know how easy it is to set up and configure on Windows. Take a look at it, and if you don't like it, there is always the option to buy a more capable UPS...
Remember, with networked UPS management the network switch needs to be on UPS power for the management software to work when power is out... :-)