|The setting can be accessed by command line, but it would be good to find why the power applet doesn't open.|
Open a command window: Start / Run and enter CMD, or Winkey - r and enter CMD, or find "open a command window" in the Accessories menu item
In the window enter powercfg.cpl. If there is an error message select it then right-click on it to Copy it, and paste it here.
If it's a corrupted .cpl, probably the next step is to run sfc /scannow from that command window. It will take some time to complete. Or just import a new .cpl from another xp install, in Safe Mode.