Replaced hard drive ribbon: still getting same errors, is it my hard drive?

Hi, i have a macbook pro mid 2009 13 inch, and it was getting stuck on the startup with the apple and spinning wheel, so i did what most people suggested and replaced my hard drive ribbon with a brand new ribbon. when i got into disk utility again and tried to erase, partition and reinstall OS X, it gave me the same error messages (POSIX reports: cannot allocate memory) . When i use disk utility to verify and repair the hard drive, it says the hard drive appears to be OK and nothing looks wrong, but i still get the errors when trying to install the OS X. Im guessing its the hard drive that is somehow broken, but id like to be sure before i start buying a bunch of parts. Thanks for reading!