<p>Given a username and a password for a domain user, what would be the best way to authenticate that user programatically?</p>

You can use some hacks to
authenticate
only.
```
```
Try
    Dim directoryEntry as New DirectoryEntry("LDAP://DomainController:389/dc=domain,dc=suffix", "username", "password")
    Dim temp as Object = directoryEntry.NativeObject
    return true
Catch
    return false
End Try
```
```
If the user is not valid, the directory entry NativeObject cannot be accessed and throws an exception.  While this isn't the most efficient way (exceptions are evil, blah blah blah), it's quick and painless.  This also has the super-cool advantage of working with all LDAP servers, not just AD.