You can fix/repair that using the Disk Utility, or reset the user's password using the original install DVD: http://support.apple.com/kb/ht1452
This could also be due to a different problem. Check your activity monitor for any processes that run higher than, say 80% at all times. It could even be a bad printer driver with a stuck print job, so maybe you should also check your print queue...
You'll have to remove a lot of different sized screws. Be careful to take care of properly placing them while doing the steps - you'll have to put them back the way you took them out. I personally placed them the way the pictures show them.
Try to experiment with the temperature of the soldering iron if you can adjust it. If it's too hot for the solder you have, it'll melt it away in a small, spectacular boil and evaporation sequence.
It is probably much easier to remove the solder with the wick, so be sure to get some. I did it without and it took forever.
Be sure to use the square hole, more to the inside, not the one between the outer shell and the little piece of metal. The Outer shell is attached to the metal.
You need to stick the end in, and it detaches if you push it upwards.
The replacement takes about 10 minutes from start to finish. The longest part is the OS Backup/Restore.
BTW, you might want to know before you begin: if you have a similar size backup hard drive that you originally have in your computer, you should "Restore" the contents of the original hard drive onto it before starting this manual, using the Disk Utility. (Yes it's called Restore. You restore the original to the backup. lol.)
Your other alternative is to get an external enclosure for your SATA Laptop hard drive that has a Firewire port on it.
After the swap, you connect either the backup drive or the original drive in the external enclosure using Firewire, then boot while pressing the Option key. The backup drive will show up (possibly as the only boot option.) (In fact, now that I think about it, you might not even have to press the Option key, since the external drive should be the only boot option unless there is something in your DVD drive...)
Boot to the backup drive, then "Restore" its contents to the new internal hard drive using the Disk Utility. You should be set with your original OS, settings, etc.