Sounds like the same problem I had. Sometimes it would boot and sometimes it would just stay on the black screen with the "Window Loading" until it got to a point it wasn't going to boot at all.
I also ran Diag on my HDD and memtest and nothing unusual pop out test came out perfect. I manually pull out some documents from my HDD hot-plugin it to a laptop. I wrote zero on the first sectors and not the whole HDD. I did a fresh W7 install and now is running great!
APC P11VNT3 -- Asus VE248H 24" -- Rosewill Destroyer -- Corsair 650TX -- Asus P8z68-V PRo -- I7 2600K -- Corsair Vengeance 8GB LP -- WD CB 2TB SATA 6.0Gb -- Asus PCE-N13 PCI Wireless Adapter