What's the difference between Windows XP Service Pack 1 (SP1) and XP SP1a?

A. One difference between these two service packs is that SP1a doesn't include the Microsoft Java Virtual Machine (JVM). If you're already running SP1, you don't need to install SP1a. If the SP1 JVM isn't installed on your machine, Windows Update won't provide any JVM installation options.

SP1a also includes two registry changes that signify that the computer is running SP1a:

• A new value in the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\SubVersionNumber registry subkey that is set to "a".
• A new "SP1a" string in the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Updates\Windows XP\SP1\Q324720\Description registry subkey.

SP1a also changes the Control Panel Add/Remove Program applet to display "Windows XP Service Pack 1a."