Don -

Here are two urls that should help:

1.  Forms Authentication with Active Directory:
http://msdn.microsoft.com/library/en-us/dnnetsec/html/SecNetHT02.asp

2.  Forms Authentication with SQL Server 2000
http://msdn.microsoft.com/library/en-us/dnnetsec/html/SecNetHT03.asp

I actually went through the AD sample and found it quite helpful.  I hope 
this helps you.

Best,

- Rich