My guess (and it is a guess) would be a bad graphics card or video driver. I'm assuming that since you said you tried it in safe mode, you can get to the point that you can select the boot type. To verify this, try using a Linux Live boot disk. If you can boot into Linux, it's the drver, otherwise it's likely the card.
"The HDD may be problematic - I had a similar issue last week and had to replace the drive.
Have you tried booting from a bootable CD or DVD? Such as a Windows disk or a Linux live CD? Being able to do that would suggest that the problem is indeed the HDD.
Edit: you mention the diagnostic mode. Are you able to get into the system's BIOS? If so, is the HDD listed there?"
