most newer models rely on internal batteries for BIOS/cmos, so if you don't see any battery buttons on your board it means that it relies on your built in battery.

replacing the main board will solve your problem.

try replugging the flex cables of the hardware that you replaced, improper connection seating in iphones causes boot loops and other problems as well as improper screw placements.

have you tried using different power brick /adaptor with the same voltage rating as the original?,

Do you hear windows chime as your laptop boots up?,if not then do this try connecting your laptop to a external monitor using the laptops' vga out, then turn on your laptop as usual and try to observe if a display comes out of the external monitor. if none, try turning it off and isolate your RAM or memory individually or much better try removing all your ram and boot up your laptop without ram if you hear a successive beep on power up replace your ram.

if the screen turns on and all the buttons work perhaps its the digitizer that needs replacement., but try to replug the connector coz sometimes when the unit got dropped some parts come loose.

try repairing your windows by using your installation disk select repair and when the command prompt appears type the following after the command prompt: chkdsk /r /p