Windows 2000, Windows XP, and Windows Server 2003 will shutdown when you start the UPS service if both of the following are true:
The UPS signal polarity is set to Negative for Power Fail/On Battery.
There is no UPS attached to the computer, or the UPS device is not working.
Starting with Windows 2000, the operating system shuts down when it cannot detect that there is power to the UPS.
If you don't have a working UPS device attached to your computer, you should select None on the UPS tab of Control Panel / Power Options, and you should make sure that the UPS service is configured for Manual startup in Administrative Tools / Services.
If your computer continually restarts, start your computer in Safe Mode.