yes, windiag is good too, I always used to use that before i found memtest86.
Anyway, so we can rule out the RAM. looks like It might well be your Hdd at fault, this was always the most likely senario.
Have you tried reinstalling yor mobo chipset drivers/software? its possible that your ATA driver might be corrupt.
Also, download the diag soft from seagate and give it a go here