I've also had this, under XP and Vista. System just keeps resetting the l/r positions on its own, it's rubbish!
The easiest way to fix it is to get your general settings as you want them with the Nvidia control panel, then use the standard windows settings (right click on background of main wallpaper, select appropriate options - in Vista it's 'personalize', set the screen left/right positions in there.
Hope this helps
BW