Locked out! PLEASE HELP!

Question

Any help would be appreciated greatly! To start, I woke my computer screensaver and it prompted me
with the login screen, as usual. However, when I typed in my password, the login screen shook as if it
were wrong. Thinking I may have typed it wrong, I typedit in again...and again...and again....and again.
Much to my dismay, it was not accepting my password. I hadn't changed it or fiddled with any of the settings
prior to this happening. So, thinking it might be confused, I held down the power button and shut it off. Rebooting it, I was prompted again with the login. This time, it no longer showed my username. Just two blank fields. I typed in my user name, which I know as well, and my password. Still that littled prompt screen shakes in refusal. Now I am not a mac expert and I only know enough to just barely get by if problems come up. Knowing I am ignorant as to how to go about fixing this, I used my sister's computer to look around for answers. I found out I could use the mac OS X installation disk to try and reset the password.
This ended in defeat as well.
I clicked on my hard drive, selected my user in the dropdown and typed in the password (the same one I've always used. (Maybe that's the problem?) Everything seemed fine and I rebooted to see if the installation disk had fixed the problem. I ran into the two blank fields again. Nothing I typed in, all passwords I would ever use and different ways of typing my name, and they still refused the info I typed in.
Booting up on the installation disk again, thinking I could try and change the password again, I was met with another horror: the dropdown said there were no users on my disk to choose from. This is where DiskWarrior comes in. I owe this software everything! I used it and found that my data was fine. I even moved my important files off onto an external hard drive just to be safe. While in there, I could even see that my user was completely intact.
Still, even after trying to rebuild my directory with disk warrior, repairing my disk and permissions with the installation disk, all of which showed problems with my disk and fixed them, I am completely locked out of my computer. I called apple support and tgey said they could try and diagnos it over the phone or I could send it in to them. Mind you, no-one I spoke to knew what the problem was, let alone having heard of it. I'm a little low on cash and I'm not sure I want to pay to have my computer looked at only to find that they don't know what to do. (My computer us almost four years old. It's a tough little ****, having gone through a lot in my care, and it's out of warrentee, meaning the cost of diagnosis would not be covered by apple.)
I need to know several things:
1. Has anyone had this problem and solved it? If so, how dud you fix it?
2. Apple seems to think it's some form of hard drive failure and suggested an archival restore or something, where the computer would reinstall its os without affecting my personal data. Chillingly, they said if that doesn't work, they recomend EARSING THE HARD DRIVE COMPLETELT. That's the last step I ever wish to take, but do you think the archival restore or whatever it's called will solve it?
3. I did some research on the net, driven to find some way of fixing it at home, and my problem might have something to do with my Keychain. Now not being able to even log in to my computer, is there somehow a wat to access the keychain and alter/fix/erase the login info there?
4. On the matter of the keychain, several people on the internet have suggested a solution on how to change or bypass the keychain by starting up the computer in single user mode and using programming code. I stress again: I am NOT very good with computers. I'm scared to death that I will do something wrong and mess up my entire computer. Even so, would you recommend I do this? Does my ptoblem sound like It has anything to do with the keychain?
Thank you in advance. I really hope someone can help or at least give me a clear path to start on.
Notes:
My computer is 13" Macbook running Tiger version 10.4.1
My computer also had a great fall as I stumbled over my cat and ever since, if I tried to lift the laptop While it is on it would shut off or restart. Other than that, my computer worked just fine. But getting it in to have that checked and fixed was on my to do list, but then this happens. Is there a possible connection?

Answer 1

(My computer us almost four years old. It's a tough little ****, having gone through a lot in my care, and it's out of warrentee, meaning the cost of diagnosis would not be covered by apple.)
Last I heard, there was "No Charge" for a genius appointment at the Apple Store. Repairing it is something else completely, but I believe they will take 15 minutes to do what they can.
Adam