Problem with the hard drive of my laptop

Hi, everybody, i having a problem with the hard drive of my laptop. When I turn on the laptop, it says ’ non-sytem disk or disk error, please insert new disk and stroke any key to start over’ or something like that. Anyway, I have no idea what to do, i’ve tried to change the hard drive, but i don’t know where it is, and i thought that maybe changing the boot sector, but i have no idea how to do that either. Every piece of information is saved, so it won’t matter if i have to lost anything, all I want it’s the laptop running again. So, thanks to anyone who might help me.

My laptop’s properties:
Compaq Evo n800v
256 Ram
1.7 Gb intel processor
30 Gb hard drive
and i don’t know of anithing else,
so thanks again.