Reenable auto updates with the third one down, "notify but don't ..."
When you're informed that there are updates available, use the custom
install, NOT the automatic one and untick SP3 and hit download for the
rest of the updates. After that you will be asked if you want to not be
informed of SP and tick it. You will only be offered SP2 updates from
now on.

Alias

How to prevent SP3 from being installed on your PC
(Windows Service Pack Blocker Tool Kit)
http://www.microsoft.com/downloads/details.aspx?FamilyId=D7C9A07A-5267-4BD6-87D0
-E2A72099EDB7&displaylang=en
(Works for 1 year from date SP3 was released)

JS

With Automatic Updates (AU) disabled, you will be offered SP3 as well as any
available post-SP2 updates available at Windows Update website
(http://windowsupdate.microsoft.com).

If you re-enable AU and select the Notify Only option, no updates (including
SP3) will download OR install without your permission. [1]

How to configure and use Automatic Updates in WinXP:
http://support.microsoft.com/kb/306525

================================
[1] cf.
http://blogs.technet.com/mu/archive/2008/07/03/upcoming-update-to-windows-update.aspx