MBP freezes (apparently) randomly.

Question

I have a MacBook Pro 15" that I purchased in 2006. It has started freezing up at (seemingly) random times, leaving me with the only option of shutting it down by holding down the power button. Usually after doing this and trying to restart, it will freeze in a similar manner again at the log in screen.
For a while I was able to just shut it down for a few hours and it would function for another hour or two, which lead me to believe the machine was overheating. But now I could have it turned off for over 24 hours and turn it back on and it will freeze at the log in screen. When the problem started I could use it for maybe an hour then it would freeze, I'd shut it down for another 1-2 hours and be able to get another hour of use before another freeze. Now I basically can't even turn it on.
The battery has been completely dead for a while, which likely is contributing to the problem. I took the battery out and attempted a restart but had the same problem. I also noticed that the battery had bulged, which is a new addition to the problem (it probably started to grow in the last week or two). I am getting a new battery and hope this will help, but I'm pretty sure I have more than just a battery problem.
I would love any suggestions on what might be done to get it running again. I don't know that I've included enough detail so please ask me questions if you think you might be able to help. Also, I know the computer is very old, but please do not suggest that I just buy a new one. If I could afford an new Mac, I would have already bought one! I do not intend to attempt any hardware fixes on my own, but if you folks can even give me an idea of what is going on for me to tell the repair people, that would be awesome. Thanks!

Answer 1

First, remove any bulging battery as it is a hazard.  Properly dispose of it.  As tjk has suggested, next run the hardware diagnostics to be sure there is no hardware failure.
How to run hardware diagnostics for an Intel Mac
Boot from your original OS X Installer Disc One that came with your computer.  After the chime press and hold down the "D" key until the diagnostic screen appears.  Run the extended tests for a minimum of two or three hours.  If any error messages appear note them down as you will need to report them to the service tech when you take the computer in for repair.
Some "common" error indicators:
SNS - sensor error
MEM - memory error
HDD - hard disk drive error
MOT - fan error
Then I suggest the following:
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, Leopard or Snow Leopard.) 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 click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the installer. Now restart normally.
If DU reports errors it cannot fix, then you will need Disk Warrior and/or Tech Tool Pro 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.