If you want to change your profiles so that this change always occurs, use
the sony vaio "power panel." In win2k and xp systems:
1) go to lower right-hand corner of screen
2) *right* click the battery icon "Power Panel"
3) left click edit/create profiles
4) left click "maximum battery life" in left-hand pane
5) double-click "lcd brightness" (I've set mine to 7...it works well)
best o' luck