I know this script can be found in a billion different places, but i wanted to turn it into a cmdlet, so here it is. the 'OU' Param will default to the root of the current domain, so be careful if you have hundereds of workstations that have not logged on in long time. Enjoy

Get-ADInactiveWorkstations.ps1

<#
.SYNOPSIS
    Queries AD DS for workstation accounts that have not logged in for a given amount of time.
.DESCRIPTION
    Queries AD DS in spcific OU and uses the LastLogonTimeStamp to determine the last time a computer account has logged on.
    The Default Value of is 180 for days a computer account is inactive.
    The Default Value of the SearchBase is the root of the domain of the currently logged on user.
.EXAMPLE
    Get-ADInactiveWorkstations
    In this example, all workstations that have not logged on within 180 days from the root of the current users domain will be returned.
.EXAMPLE
    Get-ADInactiveWorkstations -DaysInactive 90 -CsvFile C:\InactiveComputers.csv
    In this example, all workstations that have not logged on within 90 days from the root of the current users domain will be returned and then exported to a .csv file.
.EXAMPLE
    Get-ADInactiveWorkstations -OU "ou=Computers,dc=MyDomain,dc=org" -Csv "InactiveComputers"
    In this example, all workstations that have not logged on within 180 days beggining in the OU named "Computers" in the domain "MyDomain,org" will be returned and then exported to a .csv file.
.NOTES 
    Requires ActiveDirectory Module available with Remote Server Administration Tools
    RSAT 7SP1: http://www.microsoft.com/en-us/download/details.aspx?id=7887
    RSAT 8:    http://www.microsoft.com/en-us/download/details.aspx?id=28972
    RSAT 8.1:  http://www.microsoft.com/en-us/download/details.aspx?id=39296
.LINK
    http://dotps1.github.io
#>
function Get-ADInactiveWorkstations
{
    [CmdletBinding()]
    [OutputType([PSObject])]
    Param
    (
        # DaysInactive, Type Int, The number of days a computer account has not logged in.
        [Parameter(Position = 0,
                   HelpMessage = "Amount of time, in Days, that have passed since the workstations has logged on.")]
        [Int]
        $DaysInactive = 180,

        # OrganizationalUnit, Type String, The LDAP OU String to enumerate.
        [Alias("OU")]
        [Parameter(Position = 1,
                   ValueFromPipeLine = $true,
                   HelpMessage = "The top most Organizational Unit to recursvily search from.")]
        [String]
        $OrganizationalUnit = "dc="+($env:USERDNSDOMAIN).Split('.')[0]+",dc="+($env:USERDNSDOMAIN).Split('.')[1],

        # CsvFile, Type String, Location to output report in csv format to.
        [Alias("Csv")]
        [ValidateScript({ if (-not(Test-Path (($_ -split '\\')[0..(($_ -split '\\').count -2)] -join '\'))){ throw "$_ is not a valid path." } else { return $true } })]
        [Parameter(Position = 2,
                   HelpMessage = "The Fully Quallifed path to place the output of a this cmdlet into a CSV.")]
        [String]
        $CsvFile
    )

    Begin
    {
        if (-not(Get-Module -Name ActiveDirectory)) 
        {
            try
            {
                Import-Module -Name ActiveDirectory -ErrorAction Stop
            }
            catch
            {
                throw "ActiveDirectory Module is not available on this computer, and this cmdlet cannot be used.  Use Get-Help Get-ADInactiveWorkstations -Full and see the .NOTE section for links about RSAT."
            }
        }
    }

    Process
    {
        $dateTimeEnum = (Get-Date).AddDays(-($DaysInactive))

        $workstations = Get-ADComputer -Filter { (LastLogonTimeStamp -lt $dateTimeEnum) -and (OperatingSystem -notlike "Windows Server*") } -SearchBase $OrganizationalUnit -Properties LastLogonTimeStamp,OperatingSystem |
                        Select-Object Name,OperatingSystem,@{ Name = "LastLogonTimeStamp";Expression = { [DateTime]::FromFileTime($_.LastLogonTimeStamp) } } |
                        Sort-Object LastLogonTimeStamp

        return $workstations
    }

    End
    {
        if ($PSBoundParameters.ContainsValue($CsvFile))
        {
            if (-not($CsvFile.ToString().EndsWith(".csv")))
            {
                $CsvFile += ".csv"
            }

            $workstations | Export-Csv -Path $CsvFile -NoTypeInformation
        }
    }
}