Then test your RAM before buying new. I would have done the same with the harddisk. To test your RAM download
Memtest86+, unpack it and burn the ISO to a CD as image. Then boot from it and let the test run a few hours. If it comes up with errors, at least one of your RAM modules is faulty. If you have more than one remove one of the modules and do the test again to find the faulty module.
For clarification: Bluescreen 0x7b in Windows (2000 and later) means that the system can't access the harddisk. Since you already have changed the disk it is possible that it is your harddisk-controller that is faulty. But test the RAM first.