I would recommend reinstlaling OS X. If you have another mac computer, use the thunderbolt-to-thunderbolt cable to backup your data If you cannot connect directly to another computer, I have a plan for this as well, but in the interest of making this as easy to follow, message me if you need that plan.
Install or Reinstall Lion/Mountain Lion from Scratch
Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive.
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Erase the hard drive:
1. Select Disk Utility from the main menu and click on the Continue button.
2. After DU loads select your startup volume (usually Macintosh HD) from the
left side list. Click on the Erase tab in the DU main window.
3. Set the format type to Mac OS Extended (Journaled.) Optionally, click on
the Security button and set the Zero Data option to one-pass. Click on
the Erase button and wait until the process has completed.
4. Quit DU and return to the main menu.
Reinstall Lion: Select Reinstall Lion/Mountain Lion and click on the Install button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible
because it is three times faster than wireless.
thanks for the reply. a couple of noob questions:
1a. is it possible to backup my macintosh HD somehow from my windows partition? (using a USB 3.0 drive?) (if not...)
1b. can I use a thunderbolt to firewire connection for the computer to computer connection? (I have another slightly older mbp5,4 I could use).
2. just to be clear, how is it possible for me to backup my HD (even it it is cpu to cpu) if I can't get past the grey start up screen, before a reformat?
2b. when I reformat (it has been a while since I have done that), am I given a prompt to ask: "would you like to backup your HD before the reformat?"
thanks again for the help
1a: No. Since the mac boot drive HAS to be in MSC OS Extended (Journaled), windows would not be able to read from the disks.
1b: You might be able to do that, I would not be the perfect expert for that.
2a: Get an external HDD. (if you have one, back up the data on that disk and split it into two partitions, one partition with atleast 40-60 GB. Boot into the copy of OS X on the hard disk. Aside from it being painfully slow, you will still be able to use that OS to copy files from the internal drive (assuming it works) Once you have coppied all of the files you want to back up using the external OS X, then start over, unplug the hard disk, boot into the recovery partition, erase the internal disk and install Mtn Lion like normal.
2b: Doesnt matter. Follow 2a.