Enumerate EXE and DLL signatures, returning Issuer and Subject.

Get-Signatures.ps1

# .\Get-Signatures.ps1 |  Export-Csv -Path output.csv -NoTypeInformation

function Split-X500 {
    Param([string]$X500)

    $dict = @{}
    $X500 -split ', ' | ForEach-Object { 
        $item = $_.Split('=')
        $dict[$item[0]] = $item[1]
    }
    $dict
}

function Get-Signer {
    Param([string]$Path)

    $sig = Get-AuthenticodeSignature -FilePath $Path
    $cert = $sig.SignerCertificate
    [PSCustomObject]@{
        Path = $Path
        Status = $sig.Status
        # e.g. 'CN=Microsoft Windows, O=Microsoft Corporation, L=Redmond, S=Washington, C=US'
        Subject = (Split-X500 -X500 $cert.Subject)['CN']
        # e.g. 'CN=Microsoft Windows Production PCA 2011, O=Microsoft Corporation, L=Redmond, S=Washington, C=US'
        Issuer = (Split-X500 -X500 $cert.Issuer)['CN']
    }
}

Get-ChildItem "$env:WINDIR" -File -Recurse -Include '*.dll','*.exe' -ErrorAction Continue | ForEach-Object {
    Get-Signer -Path $_.FullName
}