Use User Credentials in MDT 2013 to Run Powershell Script

#UserName
$tmpuser=$tsenv:UserID
$tmpuser=[System.Text.Encoding]::Default.GetString([System.Convert]::FromBase64String($tmpuser))

#DomainName
$tmpdomain=$tsenv:UserDomain
$tmpdomain=[System.Text.Encoding]::Default.GetString([System.Convert]::FromBase64String($tmpdomain))

#set password to variable
$tmppassword=$tsenv:UserPassword

#Decode Password
[string]$tmppassword=[System.Text.Encoding]::Default.GetString([System.Convert]::FromBase64String($tmppassword))

#Convert to secure string
$mypassword=ConvertTo-SecureString -String $tmppassword -AsPlainText -Force

#format domainname\username for PSCredential object
[string]$tmpFQusername=$tmpdomain + "\" + $tmpuser

$creds = new-object System.Management.Automation.PSCredential($tmpFQusername,$mypassword)

Place the above code into your custom scripts to use PScredentials