American Power Conversion (APC) makes a lot of different UPS's. To get what is call "Basic Signaling" from the UPS to a PC you need a serial cable like this:
If you have one of the smarter UPS's like a Back Ups Pro, then you can use a smart link to get more information from the UPS into the PC:
I have tested both of these configurations with a Back Ups Pro 1000 on Windows 2000 Server.