Windows XP didn't have Aero glass and other visual effects and shadow indexing (for fast search) that probably consume extra power in Windows 7. I wonder if disabling some of those features will save some battery life.
Yes, it will save some battery life, but probably not much. On my laptop (admittedly, that was on Vista years ago) disabling Aero made a difference of about half an hour of battery life, the same went for disabling WLAN. The search indexer won't usually run on battery power anyway and instead will wait until you have a power adapter again.
Disabling Aero to save power also does not necessarily work. Aero works with Desktop Composition, which means every window's visuals exist in graphics memory and can be composited without redrawing. If you disable it and move overlapping windows the underlying windows would have to repaint since all there is is the surface you are seeing on your monitor.