Hard Disk Cannot Be Found

I always had problems with losing system memory.
I had changed the whole PC, changed the hard disk, RAM etc but in vain.
Ultimately decide to reformat a hard disk from another system, install only the minimum and standard applications (Windows 98,Office 2000, etc).
The problem now is that the PC will always have the problem of hanging. When I Reset the system with the small button on the PC, it will not be able to detect the hard disk. Only when I reboot the PC via the big button, can it detect the hard disk and carry on smoothly from there.
Is it a hard disk, cable, motherboard or software problem? How to resolve it?
Could it be due to the ActiveSync that I am running via USB?

Thanks. 🙂