I recently had this problem and came upon this websight which fixed my problem. I was only hours away from wiping the drive and giving the laptop away. Make sure you unscrew the screw in the middle near the back of the laptop. I screwed up my F3 key trying to pry off the keyboard!
Its now working like it used to, but for how long?
The model on the bottom reads , pcg-671l. 13"screen with XP home installed.