I would highly recommend you purchase "Spinrite" from grc.com

I had a similar situation where I invested in a lot of time building a desktop, and after a few weeks of use, it would fail to boot up.

I thought it was the hard drive was was ready to buy a new hard drive. But I decided to try out Spinrite, and lo and behold, the hard drive was fine, something had corrupted the OS instead.

I restored to a prior image I had saved using Acronis True Image, and it's been humming along ever since.

The $89 I paid for Spinrite paid for itself, since I didn't waste money to buy a new hard drive. Now I use Spinrite every 2-3 months to do a maintenance run on my hard drives, and it even fixed a hard drive that over-heated in a USB enclosure which works just fine now.

So do yourself a favor and try out the software to see if it really is the hard drive...