Not a rosy situation when you have an SP3 install that you cannot roll back
IMO, you can, by Removing SP3 from Windows Update, in ADD/REM PROGS, but you will have to manually Download & Install all of SP2's Critical Updates & choose which Optional Updates you want, by going to: Start / Windows Update. Ignore SP3 & choose Custom & continue with prompts.
If you don't know what the Optional Updates are, don't Download them.
You'll need to Disable Windows Automatic Update & it's prompt in the Taskbar & continue to Update Windows SP2, on a regular basis, otherwise you'll get SP3 back-again.