On OLD computer:
Method 1:
Start Calibre
Select a Book
Tap 'O' That is the folder (in the Library) where the book is in your library
Move 2 layers up (towards root)
Author
Library <<<<<<<<<<<<<<<<
Method2:
Use the OS to search for
metadata.db <<<
That folder is
a 'Library' (it may be a wrong one, which is wy I first use method1 to get the one that is active.

If method 2 turns up more (
NEVER MIX the content using
Copy ), 1) create a New Library (additional) on the new PC or 2) Merge The 2 by USING CALIBRE (Copy to Library (right-click function) or ADD books)