Login Authentication for Standard Accounts

We have a couple of vendor applications that create standard accounts.
These accounts have the same format as our domain accounts. For
example, my Windows account would be <domain>\<user_name>. The SQL
standard account would be <user_name>.

What we would like to do is change the login process to validate
against active directory either prior to or instead of checking the
SQL password. Everything else could remain the same.

This would be on SQL 2000 or SQL 2005. Any suggestions or other
insight would be appreciated.

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