Possible HDD issues? I'm stumped.

Hey guys, so I've been using my Apple Macbook 2.16 GHz Black Polycarbonate for the last three years running on OS X Tiger quite happily.
I recently decided to make the switch to Snow Leopard, so I bought a 10.6.3 disk, and went on. The installation was fine, all seemed good.

However, two days later, my macbook froze, not something it's done before. The mous was still active, but I couldn't even get to force quit. So I rebooted and went about my business. until the same happened again. So, once again, reboot.

This time when I tried to login I was told that filevault had encountered an error and I couldn't login. So I put the 10.6.3 disk in and went to disk utility where I was told that the disk couldn't be repaired and to reformat the drive. I was a little frustrated but not too much so as all the important documents were stored elsewhere.

This seemed to solve the problem, so once again, about my business. I didn't restore any of my information on to the macbook, just in case it happened again. Which it did.

This time on bootup, I was shown the "Disk not found" symbol flashing merrily at me (Folder with a ?) so back into disk utility I went. Where to my surprise, upon clicking verify, I was told all was well... If I clicked on information for the drive, it showed up as being read/write able, and bootable. However in the startup disk menu, it wasn't present.

After much fiddling, I decided to, for the last time, format the HDD and reinstall. Which happily went well. I then saw software update flashing away at me. So i decided to get the 10.6.7 combined update done, just incase it was some sort of bug causing the freezing. I pressed the restart button on the window that popped up, and the updates began to install. Shortly after, I got an error message, something along the lines of "Update failed, update could not be expanded" or something. The computer the restarted, and bang. Back to the no disk found symbol.

Does anyone have any ideas on this? I'm going to check in a moment, but it seems that it just can't find the HDD whenever it powers down.

Thanks guys, A.

Try a Hardware Test and see what it comes up with, though it does sound like your HD is on its way out.

Ugh, well i was going to, now I have another conundrum for you.
The firmware password. I know it for definite, however when I type it in. nothing happens.
If you type in an incorrect password, you stay at the same screen and the password field clears itself, however, if you type in the correct password, it just sits there in the password field being all gloating and unhelpful. The field doesn't even clear itself to indicate it's incorrect..
I've tried the whole, remove some RAM reset PRAM thing, but i don't know when it chimes cause the thing isn't booting that far..

Right, panic over with that. It was just a case of typing the correct password, then waiting for 10 minutes ensuring that nobody touched it.
This time on verifying the disk, it needed repairing, faults with the B-Tree and partition boot system. That'll have been the problem, however, I'm going to run a hardware test right away. And then try another HD 'cause I have a spare one still boxed lying around here somewhere
Thanks