How can I make my Mac shutdown a Windows PC when my UPS signals power loss?
Take a look at this SuperUser question How can I shutdown a Windows machine remotely from OS X?. It recommends installing and using the net command:

net rpc shutdown -I xxx.xxx.xxx.xxx -U username%password


You can then use a tool like Power Manager to trigger the net command when your UPS detects the power failure.



I work on Power Manager, so feel free to ask technical questions.