Reinstalling Mac off a different hard-drive

Hi,
I've had my macbook pro for about a year and a bit now and i'd been using both windows and osx until my hard drive crashed and i installed a new one (from a windows running computer.) Being lazy, i never got around to changing anything on it so i've just had windows running on it for a while. Due to a weird anomaly in my work network, only mac's can connect,so i want to install mac again, how do i reinstall mac now? I can't just reset the hard drive because it isn't a mac one, or can i?
anyway, what osx do i download and where?
any help is appreciated
Thanks

Info regarding your Mac? Year and Model? And... what version of OS X are your original install disks that came with your Mac? If no install disks, what version of OS X was originally installed?