Don't buy a new hard drive, that's a waste of your money. Well, unless you need / want one. But you don't need one, that's my poiint.
Explain in detail when the computer stops loading, do you at least get to see the Windows screen begining to boot, or does it stop after the BIOS screen? (Where a bunch of information is shown from your PC.. Speed, memory test.. That sort of thing)
Does it beep when it stops loading?
What version of windows are you using?