How to check if a Windowa account exists?

Hello All,
In a ASP.NET application, Windows Authentication is used. For a reason the administrator needs to specify some Windows accounts to make application-level groups (like Resource Pool in Project Server 2007). Therefore, it is needed to check if the Windows account really exists in a Domain or in local users. Account names are entered in DOMAIN\username or COMPUETNAME\username format.
 
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