Have you tried start up repair? Give that a whorl, as that sometimes works. However, it is acting like there could be a hardware issue. I had a similar experience with a faulty HD 4850x2 sometime back and removing it and going with another GPU fixed the issue.
Also, during the W7 install process, did you notice that it seemed to be taking a long time?
Changing from AHCI to IDE should only solve your problem if you installed windows on IDE mode and changed to AHCI afterwards (or vice versa). Windows detects and installs appropriate drivers upon instalation. Changing it after Windows is installed could therefore cause a boot problem.
If you HDD supports AHCI it's the better option thuogh since its faster. For a relatively painless fix you could try repairing by booting from the Windows CD or simply reïnstalling. For the latter option it might be enough to simply install over a current installtion leaving most things in tact but this might also leave too much in tact an not solve the problem
You might have a temperature issue. Are you sure you installed the cpu sink right? This kind of delayed crash can be caused by rapid heat rise. If you idle in the bios for a while, does it still crash at the same point when you finally boot?