Can I use my Win-7 installation disk on another computer if the original computer it came with is no longer in use?

It depends.
If you have a "retail version" of the disc, you may be able to transfer the activation license.
If it's a "system builders" disc, you may not.

If this is a computer that originally came with Windows 10, there may not be Windows 7 drivers for all its parts.

It is the type of disk that comes with Cyberpower computers.
Yes I want to downgrade a win-10 computer.I want win-10 file explorer to be more like win-7. I cant get anything done the way it is. I spend half of my time trying to figure things out.
I looked for a retail copy but could only find one for $149. I thought that was too much.
There are cheaper versions for download but I prefer the actual disk if possible.