I recently installed Windows7 on my FZ38M and I didn't notice any problem with it (yet). I haven't used it a lot with battery but I will try it.
During the install I remember that it didn't find the drivers
for Vaio Power Management. Maybe that's your problem.
Anyway, please check that you are using the appropriate "power profile" like battery mode or something like that. It should reduce the frequency of the CPU and therefore increase your battery life. WiFi takes an important slice of your battery as well.