Have you checked the disc to make sure it's not scratched or smudged ?
It obviously wasn't the hard drive that was damaged when the laptop was dropped (it might have been damaged, but there is something else in the system not working too). Try re-seating the ram (also try the installation with only one ram module installed, and/or one ram module in each slot). The problem could also be just some component or cable in the system that jarred loose on impact, but you'd have to disassemble the laptop to re-seat all the internal components...not a quick and easy job. If it's none of the above, you're probably looking at a motherboard replacement.
Just to be thorough, check the voltage output of the charger with a volt meter and try the install without battery installed.
