Win7 not booting, startup repair working. Help please!!?

Hi, the story is...win7 won't boot, comes up with options 'start normally' or use 'startup repair'.

I used startup repair and it went through once(took 3 hrs) then rebooted but didnt fix the problem. It started again and has been going for about 4 hrs.

The computer is a 6 month old Acer 5738Z and 2 days ago I just had a new hard drive, with Win7 preinstalled, put in it during a repair under warranty.

Everything seemed fine and it was faster than before, then it started to crash. I shut it down properly and today I woke up to it not starting:(

What should I do with it? I've searched and read many things hereand I think there are 2 options-

During boot press f9, f10, f11 or f12 to access system restore (i'll probably try it when startup repair finishes)...or

Hiren's boot cd (which I have but don't really know how to use it).

Details...

6 month old computer

1 day old hd with pre-installed win7

no cd's came with the computer

since using it(the new hd) i've installed firefox, miro(download program), downloaded a couple of albums from trusted sources, installed avast antivirus, uninstalled mcafee security, transferred some trusted music files from an old computer and browsed safe internet sites.

startup repair ran once but didnt fix it

startup repair is running now(4 hours and seems like its working) and i plan to leave it go overnight and take some action in the morning.

Regarding system restore and hiren;s boot cd what should I do? Or any other options.

Any advice/recommendations/suggestions/answers greatly appreciated. Thanks.