Function Test-UserCredential {
    Param($username, $password)
    Add-Type -AssemblyName System.DirectoryServices.AccountManagement
    $ct = [System.DirectoryServices.AccountManagement.ContextType]::Machine, $env:computername
    $opt = [System.DirectoryServices.AccountManagement.ContextOptions]::SimpleBind
    $pc = New-Object System.DirectoryServices.AccountManagement.PrincipalContext -ArgumentList $ct
    $Result = $pc.ValidateCredentials($username, $password).ToString()
    $Result
}

$comp_username = Get-Property -Name COMPUTER_USERNAME

$comp_password = Get-Property -Name COMPUTER_PASSWORD

$result = Test-UserCredential -username $comp_username -password $comp_password

if ($result)
{
#Enable "Next" button
}

else
{
#Disable "Next" button
}