1# If I put in a new hard drive, will I have to reinstall an operating system?
yes, a new empty hard drive will have to be formatted and the os reinstalled
reboot and hold down the Option key
Choose the “Mac OS X Installer” startup volume from the boot menu
Select “Disk Utility” and choose the hard drive you wish to format, click the “Erase” tab, and then pull down the “Format” menu and select “Mac OS Extended (Journaled)” as the type, name the drive if you wish
Click the “Erase” button and let the drive format – this is the point of no return
When finished, quit out of Disk Utility and now select the “Install Mac OS X” option from the menu
Choose your freshly formatted hard drive and install Mountain Lion
2# When you buy a mac, do they give you a copy of the OS on disc like buying a windows PC?
no, however depending on your mac, post- lion, you can use the recovery mode to download and reinstall the os
information about recovery mode
3# If the procedure much different between different flavours of macbook?
yes and no, different on different generations of MacBooks, but it is pretty straight forward and easily done link is to change ram, however look it over, and read the useful tips, the hard-drive is right next to the battery also you need to change the hard drive cover held in by screws
Are there any special tools required extra to a small philips-head screwdriver?
no not really no unless you dont have any "normal" screwdivers