There are a few ways you can list inactive (or stale) computer accounts in AD, such as via PowerShell or with a program like Hyena.
My favourite way to list inactive computer accounts is with dsquery. This is because you can pipe the output of a dsquery into another query or text file, which you can then use for something else.
To list computers that have been inactive in AD for 3 months (90 days), use the below dsquery command in a command prompt window on a domain controller or PC on the domain with RSAT tools installed:
dsquery computer -stalepwd 90 -limit 99999
If you want, you can pipe this out to a text file but adding > C:\FileName.txt to the end of the command.