Convert SID to User Name using PowerShell
Create a SecurityIdentifier object and translate into a NTAccount:
1: $objSID = New-Object System.Security.Principal.SecurityIdentifier
1: ("S-1-5-21-3496554487-1900509739-3140548133-500")
2: $objUser = $objSID.Translate( [System.Security.Principal.NTAccount])
3: $objUser.Value