I'm not sure how to explain this adequately, but hopefully you have enough technical knowledge to understand what I'm saying:
When you boot from a cold-start, does the BIOS recognize your harddrive (i.e. does it show up in the list of devices on the POST screen)?
Normally you'll see something like this:
SATA 0 none
SATA 1 none
SATA 2 none
SATA 3 none
IDE Primary/Master .......... SEAGATE XYZ123
IDE Secondary/Slave ........ Lite-ON DVD Drive (or whatever your drive is)
If you boot your computer and you don't see you harddrive in that list, the harddrive might not be initializing properly.
If you listen to your computer as it boots, are you able to hear the harddrive spin-up when the computer first powers on? Does it not spin-up until after the "replace system disk error" appears? Its hard to tell the harddrive from fans, so this might not be easy.
Does the BIOS ever display a message about how a new device was detected and its updating your VMI/HMI Tables (or something like that, I don't remember which acronym it is exactly). Does it show this when you've changed no hardware in the system?
I'm guessing one of two things here:
1) the motherboard is having BIOS issues and is losing the configuration of your system, and therefore, it keeps trying to boot from a CD or a non-existent SATA device. It might also talk about how it found a new device the first time it booted, but not when you reset.
2) The harddrive is old and not initializing fast enough. 160GB sounds old, how long have you had it? Its possible the harddrive is not initializing right-away, and the BIOS is therefore not seeing it when it first boots up. If you hear the harddrive spin-up after the BIOS POST screen, this could be a problem. The reason it works after a reset is because a reset via the button or key-press does not cut system power, so once the harddrive is initialized, it stays initialized until it powers down when you shut-off the machine. If this were the case, the BIOS would fail to report the existence of your harddrive the first time it booted, but should show it when you reset.
I'm going to guess that its your older harddrive and not your newer motherboard (but I could be wrong). Unfortunately, the only solutions I know are to have the motherboard looked at by Gigabyte, or to replace the harddrive (I'm guessing its not under warranty anymore).
If you replace the harddrive, you can maybe just backup your current drive to your new drive, and avoid the need to completely re-install windows, but that's a topic for another section of the forum.