It might not be the harddrive giving you the problem...

My first thought because of the rebooting during start-up is that you have some corrupted driver files. To correct this make sure that you update all the drivers for your hardware and applications... This is really true if you are using an older bunch of software or hardware from the windows 95/98 computers on an XP computer.

The second thought would be that you have a virus/trojan/spyware/malware problem. Run all your protection programs and make sure that your system is clean

Start > control panel > admin tools > event viewer.
check to see if there are any warnings from within all 3 areas and then highlite the warning and double click for a more detailed look at what or where the warning originated.