Hello.
If you are using Windows, then the default data directory for Bitcoin Core should be in either:
C:\Users\YourUserName\Appdata\Roaming\Bitcoin (Vista and 7 / 8 / 10)
C:\Documents and Settings\YourUserName\Application data\Bitcoin (XP)
If you are using Linux, then it should be located in:
~/.bitcoin/
The file that you need to copy is called 'wallet.dat'.
We have a Bitcoin Core wallet which is taking FOREVER to synchronise with the network on the old PC.
This is because Bitcoin Core is a full wallet, i.e. it requires you to download and verify the whole blockchain.