Well, SP1 is the first service pack for XP, SP1a was later released to address a few compatibility issues that came up with some systems. If you already have SP1 installed and everything seems to be working fine, you don't need SP1a. You will want the upcoming SP2 due out this summer sometime. I've already seen beta versions floating about, but I wouldn't use that on anything but a spare system myself.
What is the first update for XP. Is it Service Pack 1 or 1A?