problem with ups

no, not the delivery company. i got an ups for my sister because the house she was in had electricity problems and it was making her computer unstable. but now she has moved and i took the ups back : ) it does what its supposed to in that i can pull the plug and the computer keeps going. only, i cant get the software part to work, its supposed to shut the computer down when it switches to battery. for some reason whenever i install the software or choose generic ups in win xp, it shuts the system down. i have tried many different versions of the software. each time i install the software my computer shutsdown when setup is complete, then when i reboot it shutsdown again. there are no indicator lights showing a problem. any one got any ideas? returning it is out of the question.

how do you shoot the devil in the back? what happens if you miss? -verbal