yeah, if it fell and still turns on, it's like the HDD. somethign with moving parts etc.
you break a silicon board and it doens't turn on anymore
your hdd is probably in a forced PIO mode, which runs at ~3-4MB/s instead of ~100MB/s. you'd experience constant slowing or pausing of the entire system while runing. even just oving mouse across desktop it may slow down intermittently. the os isn't operating well due to the hdd slowly reading something into RAM. - one possibility
you can use a free memtest from microsoft too.. that will tell you if your RAM is funcitoning properly.
troubleshoot exactly what the problem is so you don't waste money. the fact it turns on after a fall is a good sign, lol, really.