.Net Runtime Optimization Service Is Turning My System Unstable

I can't answer for 6.0 but I have no problems whatsoever with 2003, 2005 and 2008.

If you're using .net 3.5 use this code instead.

To authenticate a user:

PrincipalContext adContext = new PrincipalContext(ContextType.Domain);

using (adContext)
{
     return adContext.ValidateCredentials(UserName, Password);
}

If you need to find the user to R/W attributes to the object do this:

PrincipalContext context = new PrincipalContext(ContextType.Domain);
UserPrincipal foundUser = 
    UserPrincipal.FindByIdentity(context, "jdoe");

This is using the System.DirectoryServices.AccountManagement namespace so you'll need to add it to your using statements.

If you need to convert a UserPrincipal object to a DirectoryEntry object to work with legacy code you can do this:

DirectoryEntry userDE = (DirectoryEntry)foundUser.GetUnderlyingObject();