If possible run a system test from bios for your system RAM,or temporarily replace the RAM to troubleshoot.
I've come across this exact problem on an HP Laptop running Vista Ultimate. Did everything I thought would work, replaced/tested hard drive, made multiple attempts to install operating system both XP MC SP2 and Vista. HP serviced and found the RAM chips were defective, I've never seen it before, but they said it was caused by a power spike of some sort. This is very rare since the computer is constantly conditioning the power supply through the laptop battery and the laptop had been working fine for 8 months with no prior issues. This might help, if not it's just one more thing to rule out.