OldComputers.ps1

import-module activedirectory  
$DaysInactive = 90  
$time = (Get-Date).Adddays(-($DaysInactive)) 

# Get all AD computers with lastLogonTimestamp less than our time 
Get-ADComputer -Filter {LastLogonTimeStamp -lt $time -and OperatingSystem -notlike "*server*"} -Properties LastLogonTimeStamp,OperatingSystem | 

# Output hostname and lastLogonTimestamp into CSV 
select-object Name | export-csv C:\OLD_Computers.csv -notypeinformation