You can use the POWERCFG.EXE program to configure power management. Power management settings prior to logon are a little dicey. I'd assign an AD startup script with the following in it to the computer:
for /f "usebackq tokens=2*" %i IN (`powercfg /query ^| find "Name"`) DO powercfg /change "%i" /standby-timeout-ac 0
That will determine the currently-selected power configuration profile name, and then assign the standby timeout when on AC power to 0 (which means "never").
If you can't run it as an AD Startup Script, you could try running it as SYSTEM by opening a command-prompt and doing an:
AT (time now + one minute) /interactive cmd
That will cause a SYSTEM command prompt to open in one minute (or less). If it doesn't open you probably didn't get it done in time. You can do an "AT" to see the queued scheduled jobs and see if it's set to "tomorrow" (meaning that you didn't make it in time).
I never remember if setting a power setting after logon, and then rebooting, causes that power setting to "stick" for the .DEFAULT user or not. It's a mess in Windows XP-- I know that. It gets better in Vista and beyond w/ group policy-based management of power settings.
If you want to get into a group-policy-based power management regime for your Windows XP machines in general, have a look at the US EPA's EZ_GPO power management tool: http://www.energystar.gov/index.cfm?c=power_mgt.pr_power_mgt_ez_gpo