LDAP Authentication From .NET Using Proxy User?

We want to use a "proxy user" to connect to an LDAP server (Active Directory, Novell, or otherwise) and then make sure that the user trying to log into the application has typed in an acceptable user name and password. I have got the code for connecting to LDAP just fine, but I'm at a loss as to how to check the user name and password. Can you do this through an LDAP query?

Here's the guts of my code so far:

Public Function Authenticate(ByVal UserName As String, ByVal Password As String)
Dim LDAPServer As String = ConfigurationManager.AppSettings("LDAPServer")
Dim proxyUsername As String = ConfigurationManager.AppSettings("LDAPProxyUser")
Dim proxyPassword As String = ConfigurationManager.AppSettings("LDAPProxyPassword")
Dim entry As DirectoryEntry
entry = New DirectoryEntry(LDAPServer, proxyUsername, proxyPassword)
'This performs the LDAP authentication'
Dim obj As Object = entry.NativeObject
Dim search As New DirectorySearcher(entry)
search.Filter = String.Format("(SAMAccountName={0})", UserName)

'How do I check the password now?'

Dim result As SearchResult = search.FindOne()

If result Is Nothing Then Throw New Exception("Unable to find SAMAccountName")