$cred = Get-Credential #Read credentials
 $username = $cred.username
 $password = $cred.GetNetworkCredential().password

 # Get current domain using logged-on user's credentials
 $CurrentDomain = "LDAP://" + ([ADSI]"").distinguishedName
 $domain = New-Object System.DirectoryServices.DirectoryEntry($CurrentDomain,$UserName,$Password)

if ($domain.name -eq $null)
{
 write-host "Authentication failed - please verify your username and password."
 exit #terminate the script.
}
else
{
 write-host "Successfully authenticated with domain $domain.name"
}

function Test-Credential {
    <#
    .SYNOPSIS
        Takes a PSCredential object and validates it against the domain (or local machine, or ADAM instance).

    .PARAMETER cred
        A PScredential object with the username/password you wish to test. Typically this is generated using the Get-Credential cmdlet. Accepts pipeline input.

    .PARAMETER context
        An optional parameter specifying what type of credential this is. Possible values are 'Domain','Machine',and 'ApplicationDirectory.' The default is 'Domain.'

    .OUTPUTS
        A boolean, indicating whether the credentials were successfully validated.

    #>
    param(
        [parameter(Mandatory=$true,ValueFromPipeline=$true)]
        [System.Management.Automation.PSCredential]$credential,
        [parameter()][validateset('Domain','Machine','ApplicationDirectory')]
        [string]$context = 'Domain'
    )
    begin {
        Add-Type -assemblyname system.DirectoryServices.accountmanagement
        $DS = New-Object System.DirectoryServices.AccountManagement.PrincipalContext([System.DirectoryServices.AccountManagement.ContextType]::$context)
    }
    process {
        $DS.ValidateCredentials($credential.UserName, $credential.GetNetworkCredential().password)
    }
}

#Get credentials
$credential_ok = 0
while ($credential_ok -ne 1)
{
    $credential = get-credential
    $result = connect-qadservice -service *domain_name* -credential $credential
    [string]$result_string = $result.domain
    if ($result_string -eq "*domain_name*")
    {
        $credential_ok = 1
        #authenticated
    }
    else
    {
        #failed
    }
}
$username = $credential.username
$password = $credential.GetNetworkCredential().password

$date = get-date
Add-Content "c:lbinInstall_log.txt" "Successfully authenticated XP script as $username $date"

param([string]$preloadServiceAccountUserName = "")

function HarvestCredentials()
{

        [System.Management.Automation.PSCredential]$credentialsOfCurrentUser = Get-Credential -Message "Please enter your username & password" -UserName $preloadServiceAccountUserName

        if ( $credentialsOfCurrentUser )
        {
            $credentialsOfCurrentUser = $credentialsOfCurrentUser
        }
        else
        {
            throw [System.ArgumentOutOfRangeException] "Gui credentials not entered correctly"
        }

    Try
    {


        # validate the credentials are legitimate
        $validateCredentialsTest = (new-object System.DirectoryServices.DirectoryEntry ("WinNT://"+$credentialsOfCurrentUser.GetNetworkCredential().Domain), $credentialsOfCurrentUser.GetNetworkCredential().UserName, $credentialsOfCurrentUser.GetNetworkCredential().Password).psbase.name
        if ( $null -eq  $validateCredentialsTest)
        {
            throw [System.ArgumentOutOfRangeException] "Credentials are not valid.  ('" + $credentialsOfCurrentUser.GetNetworkCredential().Domain + '' + $credentialsOfCurrentUser.GetNetworkCredential().UserName + "')"
        }
        else
        {
            $t = $host.ui.RawUI.ForegroundColor
            $host.ui.RawUI.ForegroundColor = "Magenta"
            Write-Output "GOOD CREDENTIALS"
            $host.ui.RawUI.ForegroundColor = $t
        }
    }
    Catch
    {

        $ErrorMessage = $_.Exception.Message
        $FailedItem = $_.Exception.ItemName
        $StackTrace = $_.Exception.StackTrace

        $t = $host.ui.RawUI.ForegroundColor
        $host.ui.RawUI.ForegroundColor = "Red"

        Write-Output "Exception - $ErrorMessage"
        Write-Output "Exception - $FailedItem"
        Write-Output "Exception - $StackTrace"

        $host.ui.RawUI.ForegroundColor = $t

        throw [System.ArgumentOutOfRangeException] "Attempt to create System.DirectoryServices.DirectoryEntry failed.  Most likely reason is that credentials are not valid."
    }

}


Try
{

    HarvestCredentials

}
Catch
{
    $ErrorMessage = $_.Exception.Message
    $FailedItem = $_.Exception.ItemName
    $StackTrace = $_.Exception.StackTrace

    $t = $host.ui.RawUI.ForegroundColor
    $host.ui.RawUI.ForegroundColor = "Red"

    Write-Output "Exception - " + $ErrorMessage
    Write-Output "Exception - " + $FailedItem
    Write-Output "Exception - " + $StackTrace

    $host.ui.RawUI.ForegroundColor = $t

    Break
}
Finally
{
    $Time=Get-Date
    Write-Output "Done - " + $Time
}

.TestCredentials.ps1 -preloadServiceAccountUserName "mydomainmyusername"