MS Gina and Vista Credential Provider

Create Custom Login Experiences With Credential Providers For Windows Vista - http://msdn.microsoft.com/en-us/magazine/cc163489.aspx

Customizing GINA, Part 1 - http://msdn.microsoft.com/en-us/magazine/cc163803.aspx

Customizing GINA, Part 2 - http://msdn.microsoft.com/en-us/magazine/cc163786.aspx

Debugging a Windows Vista Credential Provider -
http://msdnfan.blogspot.com/2008/04/debugging-windows-vista-credential.html

How to implement your own Vista Credential Provider -
http://www.jwsecure.com/dan/2006/12/13/write-your-own-windows-vista-credential-provider/