Noticed this post hasn't seen any answers yet, so lets take it systematcally. All of this is based on you having already swapped memory modules around and run with only one at a time to test if either of them were bad and causing lockups.
1) Enter BIOS and see if BIOS sees the HD, if not then you most likely have a bad HD and need to purchase a new one adn do a clean install.
2) If HD is good in BIOS and optical drive is recognized insert recovery disk into optical drive and change boot order then save and exit rebooting from recovery disk (or recovery partition if the HD still gives you that option which I am assuming it hasn't.
3) Assuming you need no data from current HD as part of recovery / reinstall reformat HD (long format) and reinstall OS. If you have a problem during this process it is likely that your HD has problems with the platters or read head, time to get a new hard drive and do a clean install.
