If you have an incomplete update to XP SP1, you will need to roll that back and then update to SP2, which includes all of the updates from SP1. There will then be quite a few critical updates that you will need to install with a fair number of reboots. Since your problem involves getting updated to XPSP2, it may be worth formatting, reinstalling XP and going right to SP2 and the updates following via Windows Update and then install your applications, including OneCare.
-steve