bad SP2 install

I had installed SP2 on my notebook but when my school botched installing
Symantec I had to "rescue" (instead of reinstall) XP Professional.
It looks like SP2 is no longer installed but the auto updates says otherwise.
In the mean time explorer.exe crashes everytime I access the control panel.
My computer also takes 3 minutes to boot up now.
Do I need to manually uninstall SP2 and reinstall it? How can I do this?

Thanks

Roaf