Microsoft Knowledge Base Article 883575 contains the following summary:

This article discusses the known issues with using Microsoft Outlook Web Access on a Microsoft Windows XP Service Pack 2 (SP2)-based computer. The topics are discussed in the following order:
Use of Outlook Web Access on a Windows XP SP2-based computer.
Existence of known issues.
Loss of functionality of the Outlook Web Access S/MIME control.
Existence of miscellaneous changes.