Six bulletins in total, including the four critical fixes, will be released, according to Microsoft's advance advisory notification.
"Critical" is the most severe ranking Microsoft assigns to security flaws. That classification typically indicates that a system can be compromised remotely with little interaction required by the user.
Specifically, Windows XP Service Pack 2, Windows Server 2003, IE 5.01 running on Windows 2000 with SP4, Outlook Express and Windows Mail in Vista are among the affected software.
Ironically, Microsoft has touted Vista as its most secure version of Windows to date, but even last April the software giant had to .
The security update is designed to address Windows Mail in Windows Vista and Windows Vista x64.