Making laptop not increase/decrease screen brightness when changing power source
Solution 1:
Look at your power management settings (try Control Panel | Power) or your laptop may have a manufacturer-supplied program to handle this. Usually these let you create and/or select power management profiles, which can include screen brightness settings.
Solution 2:
My XP laptop is a Lenovo; they do supply a power manager that has quite few more options than XP's control panel. I had a look in the registry to see what's there and found this:
HKEY_CURRENT_USER\Control Panel\PowerCfg\PowerPolicies\11
(the finall number will vary depending on which policy you've chosen, apparently)
Interesting entries (all REG_SZ strings, not DWORDS)
DISP_BRTNESS_AC=7
DISP_BRTNESS_DC=1
BRTNESS_WHEN_IDLE_AC=7
BRTNESS_WHEN_IDLE_DC=1
BRTNESS_IDLE_TIMER_AC=0
BRTNESS_IDLE_TIMER_DC=0
You might be able to add these entries to your existing profile to get what you're after.
Solution 3:
Some laptops have a screen dimming option in the BIOS. If this option is set to dim on battery, then no amount of fiddling in the OS will turn it off.
I had a Lenovo X61 that did exactly this. No idea about the Samsung Series 9. That's a "Designed for Windows 7" laptop, anyway. While XP may be supported, it might not be supported well.