I have installed a HGST 1TB Sata ex Laptop into an ASUS P5 KPL-CM - Set BIOS to BOOT from this Drive on SATA 1 - When it starts BIOS Runs and Posts, but Cursor sits blinking on a Blank Screen - NOT BOOTING - WAS Working correctly in Laptop - It was removed to Clone to a SSD.
You can not expect to boot windows from a different pc. With different pc be prepared to reinstall windows.
If you try to avoid reinstall you get several issues:
boot mode compatibility - system doesn't boot (you have this now),
drivers incompatibilities - bsods, crashes, bad performance,
windows activation issues - can not activate windows.
If I had to guess - windows on laptop was installed in UEFI mode.
Your ultra old pc (LGA775 G31 chipset) doesn't support UEFI boot.
In this situation you get blinking underscore on screen, when trying to boot UEFI windows in legacy mode.