Well, for sure you want to take the 70-220 Designing Secure Windows 2000 network exam. You should also bone up on the many good Windows 2000 security books out there, especially the Jeff Schmidt and Phil Cox Windows 2000 Security Handbook offerings (note: these are two separate titles, from two different publishers). I'd also recommend the Northcutt/Novak Intrusion Detection, 2e, book and the "Hacking Exposed" 2e book as basic tools. After that, if you decide you want to get certified, the SANS GIAC and ICSA certs will probably meet your needs best.
This was first published in February 2001