|...installed new Power supply and motherboard as an upgrade, and now it doesnt work as the boot device like it used to so I installed windows on a seperate hard drive and it still wont show up...|
To clarify - this is a desktop/tower system not a laptop.
Presuming so and to clarify the situation a little further...
You replaced the PSU and motherboard.. but not the hard drive; and now the system won't boot up from that drive?
Did it even spin up etc. and show any signs of trying to boot up as it did previously?
How did you connect the first hard drive to the new motherboard? Did the first/original drive show up in the bios settings for the new motherboard?
You installed another hard drive and installed Windows to that? Does that hard drive boot up OK?
The version of Windows you have - is it Retail or OEM; and is it win XP, or 7, or 8x?
Been a while since I swapped a hard drive from one motherboard to another... but I seem to recall that generally with Windows XP onwards.. you can't simply do that and expect it to boot up as before.. A repair installation at least might resolve it...?