Macbook will not start.

Are you saying that you get a gray screen with the dark Apple logo but nothing more? Or are you saying you get to the login screen then nothing happens? The latter would be a dialog with the "Starting Mac OS X" message and loading bar. If the latter then you could try this:

You will need to type some Unix commands. If you are not comfortable with this, I don't know of anything other than a re-install. But if you are careful, you should be OK. I recommend you print this out in a largish mono-spaced font so you don't miss any spaces (or add extra ones). Note that case is important.

Be careful. Some of these commands are dangerous, since you are going to be root.

Start up in Single-User mode by restarting the computer. After the chime press and hold down the COMMAND-S keys until you see white text on a black background. When this has finished you will see a prompt ending in '#', although there may be other messages. Enter the following commands after the prompt:

/sbin/fsck -fy

Press RETURN. Wait a few seconds for 8-10 lines of output. If the last line says repairs were carried out, repeat this command until you get a message 'The volume <yourdiskname> appears to be OK'. Then continue with:

/sbin/mount -uw /
cd /Library/Preferences
rm com.apple.loginwindow.plist
rm com.apple.windowserver.plist
cd /Library/Caches
rm -r *
cd /System/Library
rm Extensions.kextcache
cd /System/Library/Caches
rm -r *
reboot

Press RETURN after each command.

This should now take you to a proper login screen after the normal boot sequence. You should then Repair Permissions by using Disk Utility (in your /Applications/Utilities folder).

If the above is not successful then you can try this:

How to Perform an Archive and Install

An Archive and Install will NOT erase your hard drive, but you must have sufficient free space for a second OS X installation which could be from 3-9 GBs depending upon the version of OS X and selected installation options. The free space requirement is over and above normal free space requirements which should be at least 6-10 GBs. Read all the linked references carefully before proceeding.

1. Be sure to use Disk Utility first to repair the disk before performing the Archive and Install.

Repairing the Hard Drive and Permissions

Boot from your OS X Installer disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Installer menu (Utilities menu for Tiger.) After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported, then quit DU and return to the installer.

If DU reports errors it cannot fix, then you will need Disk Warrior (4.0 for Tiger) and/or TechTool Pro (4.5.2 for Tiger) to repair the drive. If you don't have either of them or if neither of them can fix the drive, then you will need to reformat the drive and reinstall OS X.

2. Do not proceed with an Archive and Install if DU reports errors it cannot fix. In that case use Disk Warrior and/or TechTool Pro to repair the hard drive. If neither can repair the drive, then you will have to erase the drive and reinstall from scratch.

3. Boot from your OS X Installer disc. After the installer loads select your language and click on the Continue button. When you reach the screen to select a destination drive click once on the destination drive then click on the Option button. Select the Archive and Install option. You have an option to preserve users and network preferences. Only select this option if you are sure you have no corrupted files in your user accounts. Otherwise leave this option unchecked. Click on the OK button and continue with the OS X Installation.

4. Upon completion of the Archive and Install you will have a Previous System Folder in the root directory. You should retain the PSF until you are sure you do not need to manually transfer any items from the PSF to your newly installed system.

5. After moving any items you want to keep from the PSF you should delete it. You can back it up if you prefer, but you must delete it from the hard drive.

6. You can now download a Combo Updater directly from Apple's download site to update your new system to the desired version as well as install any security or other updates. You can also do this using Software Update.

Not they won't affect your memory, but if you enter a command incorrectly it's possible to erase files you don't mean to. So I suggest you write them down or print them out first so you won't make a mistake.

You aren't reading carefully. There is a space after 'cd'. Read carefully as there are spaces in the commands.

Just answer "y". Did you startup in single-user mode per my procedure?

I don't think you quite understood. Single-user mode doesn't mean you are the only one who uses the computer. It is a form of startup as described in my earlier post:

Start up in Single-User mode by restarting the computer. After the chime press and hold down the COMMAND-S keys until you see white text on a black background. When this has finished you will see a prompt ending in '#', although there may be other messages. Enter the following commands after the prompt: