If I remember correctly you purchased an APC UPS. My situation seems similar.
On one machine (XP) I have an old BackUPs Pro 420 which shows up in Device Manager under the heading Batteries. The UPS service is listed as enabled and Automatic. Connection to PC is serial.
On another machine (also XP) I have a fairly recent BackUps CS 500. This one does not show up in device manager at all and in the system tray I have an icon with a yellow exclamation mark and a tool tip text that says the PC is unable to communicate with the UPS. The UPS service is listed as manual. Connection to the PC is USB.
Not being listed does not prevent the UPS from becoming active (ie taking over the power supply) and warning you with the loud beeps if there is an interruption to the power, but it will not do an unattended shutdown.
I've been meaning to talk to APC for ages (I have found them extremely helpful in the past) about this as I think it may be an issue with the latest version of Powerchute, or with the USB connection. Now that you have prompted me, I'll aim to contact them next week and will either post the response to this thread (if I can find it!) or start another.