Exchange 5.5 Web Client Hotfix 2655.77!
This patch also includes a fix for the issue discussed in http://www.microsoft.com/technet/security/bulletin/MS01-057.asp . Visit that page for more information regarding that issue.
Download Exchange 5.5 Web Client Hotfix 2655.77 for Windows NT 4.0 & 2000 Exchange 5.5 SP4 must be already installed