The Core Exams have a reputation for being the most difficult of all MCSE requirements (Core Exams don't apply to MCSA as direct requirements, so they're not covered for that credential). For Windows 2000, choices include 70-219 (directory service infrastructure), 70-220 (security), 70-221 (network infrastructure), 70-226 (highly available Web solutions), plus 70-297 (Active Directory and network infrastructure) and 70-298 (security);...
for Windows 2003, only 70-297 and 70-298 apply.
The newer exams -- 70-297 and 70-298 -- come most highly recommended because they cover Windows Server 2003 as well as Windows Server 2000 and apply equally to both platforms. The default choice is 70-297, and 70-298 applies primarily to those seeking security specializations or for whom security is a big job component. That said, those still working in Windows 2000-only environments should consider 70-219 as their default choice because of the vital role that directory services play in modern Windows environments and pick another Core Exam (70-220, 70-221, or 70-226) only if its focus coincides with primary job responsibilities.
10 PREP STEPS TO THE MCSA AND MCSE CERTIFICATION
Step 01: Choose your target Windows version
Step 02: Establish exam choices and order
Step 03: Deal with the desktop first
Step 04: Security options may appeal
Step 05: Digging into server and admin requirements
Step 06: Complete outstanding electives (optional)
Step 07: Completing the Core Exam (MCSE only)
Step 08: Prepping for individual exams (The self-study prep approach)
Step 09: Prepping for individual exams (The class-driven prep approach)
Step 10: Best exam prep practices