mac OSX won't boot and now the flashing folder

Computer Info:
Macbook Pro 15"
Mac OS X 10.4.7
2GHz Intel Core Duo
100GB Serial ATA drive

Here's the story, bear with me.

Last week, I powered on my computer after having properly shut it down the night before...after the startup chime, the apple icon and progress indicator came on...that's it. My login doesn't show up, it won't boot into the os, and basically froze at the grey apple icon screen. I tried everything.

I ran disk utility from the macosx installation cds...ran the hardware test...s.m.a.r.t was dandy...everything was fine. I went into single user mode, i got something like this...disk0s2: 0xxxxxxxxxx (UNDEFINED). I tried to do a safe boot, it won't even let me.

It just wouldn't boot into the os no matter what i tried, so finally i gave in and reinstalled the os from the installation cds. I back up my data, so it wasn't a complete tragedy to reformat.

However, just last night as I was browsing the internet, I notice that my computer was lagging. Trying to save images from the internet onto my desktop was taking way too long and the progress indicator would run for seconds before the "save" dialogue came on. I couldn't even drag the image straight to my desktop like i use to. So I decided to do a restart to see if that would cure the lag.

Big mistake. Now the grey flashing folder icon with the question mark comes on instead of the apple icon. Ran disk utility from the installation cds again. Now nothing even shows up in disk utility. No, not that the hd isn't found, but disk utility isn't even working! The progress indicator just runs and runs and runs...it doesn't even stop to show me the image of the install cds. And the only way to stop this was to force power off with the power button.

I can hear the clucking and clicking in the hard drive while the computer was lagging. Is it my hard drive? Is it dead? Do I need to buy a new one, take the laptop apart, and install it? I read several forums, but no one has my exact case. I understand if it's just my hard drive I'm willing to replace it, but should I worry about my motherboard too? I heard some people had their ram fail too, how do i check that and to see if any other hardware is failing? I want to make sure I cover all my bases so I can have it all checked at once and don't have this problem ever again.

I need to know all the possible causes with this. Thank you so much for your time and expertise.