Looks like Citrix has a workaround, but they are still looking for a fix.
Users connecting with ICA sessions receive a blue background and the Windows GINA does not display after installing Microsoft Rollup 1 for Windows 2000 SP4 on Metaframe XP and MPS 3.0 Servers. The console might be unresponsive and have high CPU utilization within the winlogon.exe process.
Issue occurs after applying the Microsoft Update Rollup 1 for Windows 2000 SP4 (KB article ID 891861) to Metaframe XP FR3 Service Pack 4 servers and MPS 3.0 servers.
Read the workaround in CTX107051.