SP1 vs SP1a
- Windows XP Service Packs -
In short - SP1 and SP1a are identical except that SP1a does not include a JAVA VM. That's it !!! As of July 2004, Microsoft released SP2.
If you installed XP, then installed SP1a - and JAVA sites on your IE browser have stopped working - see Fix Your JAVA.
The JAVA War
Initially, with SP1 - Microsoft wanted their browser to support Sun's version of JAVA, so included a Java VM (Virtual Machine) to run the code.
But with SP1a, Microsoft was forced to drop the JAVA VM. Some say that is what MS wanted, but that's another story. In short, here is what Sun requested as part of the civil lawsuit against Microsoft:
they did not get approval to force Microsoft to include the Sun JAVA code as part of Windows XP. However they did win on the the second bullet - an injunction was issued to force Microsoft to remove the JAVA VM with Windows XP.
Each company has their own side of the story . . .