OSx not recognized. Invalid node estructure error but Windows 7 partition works perfectly. All tests done but still don't know what is wrong

Question

I have an Imac Snow Leopard 10.6.3 1TB hard drive , it has a partition of 100gb for Windows 7 using Bootcamp (only for games)
The computer was kind of slow one night. The next day it was so terrible slow that I decided to restart it, but it never showed OSX again. If I try to boot into single user mode it shows Windows Boot Manager. Even booting with alt only shows Bootcamp Windows. Is like OSX dissapeared.
I tried Disk utility and it showed an "inlavid node structure" error, "Disk Utility cannot repair this disk"
I also used Diskwarrior, the test was relatively fast and showed this at the end "Disk Warrior has successfully built a new directry for the disk. The new directory cannot replace the original because of disk Malfunction. Disk Malfunction is a failure of or damage to any mechanical component of the disk or any component connected to it"
I also did a S.M.A.R.T. test from diskwarrior which turned out ok
I used Apple Hardware test, (booting with D and the Applications DVD). It goes through several things, even logic board but when it reaches "testing in progress" almost at the end it gets stucked. I have tried the simple and complete test in many different ways, with mouse, without, everything disconnected, it doesnt matter,  clicking the "Stop Testing" button or using cmd+period (as instructed on screen) doesn't stop the test. The "Total Time Testing" field also stops counting. I have to force shut down with the power button every time.
But here comes the most odd part of this story. The computer now almost always boots in the Windows 7 partition which works perfectly. I can chat, use viderocamera, save, delete, listen to music, watch videos, etc.... I even enter from Windows all my MAC Archives and backuped almost the 900Gb I had with no problem (great thing to use Bootcamp, never imagined it will be this useful).
So my question is, WHATS WRONG? The hard drive? the logic board? the fan? I need to know in order to buy the replacement. I have no warranty anymore and would like to fix it myself.
Please help me understand what is going on

Answer 1

RESOLUTION:
I erase the drive completely following the security measure "Zero Out Data" and then reformat the drive. According to what I read, by following the zero out data procedure, the computer checks the drive and isolates possible structural errors in the disk ( don´t know if it is true) I had everything already backup through Windows as explained originally.
After I reformat the drive everything is working OK. I have done the tests again (Disk Utility and Disk Warrior) that showed the drive to be in perfect order. So probably the error was caused by lack of sufficient free space in the disk ( 30 highly fragmented GB for a 1TB computer) that made OS X incapable of handling new information and was deleting key elements (?) in short a fragmented and saturated disk. However still do not understand why most tests could not complete or showed info relating to structural damage and now the same tests are saying everything is OK...
So if anyone goes through something similar, try erasing and formatting the drive, see how it goes.
Thank you very much baltwo!