A few PowerShell lines in order to get the SID of a local user to use it.
First the script targets the user you want to known his SID. Then it translates the result in a way to get a readable object. Finally it returns the result.
$objUser = New-Object System.Security.Principal.NTAccount("BladeLogicRSCD")
$strSID = $objUser.Translate([System.Security.Principal.SecurityIdentifier])
$strSID.Value