I'll keep this brief. You've likely read how I commonly use procmon to debug installer problems. Today I had a virgin (unpatched) XP SP2 machine. I reinstalled XP SP2 from the Dell OEM CD that came with it. I wanted to apply SP3 to it right away before anything else. About halfway through the install, I got an "Error: Access is Denied" message box, with no further information and no recourse to fix.
Long story short, the amazing procmon told me that the the culprit was a registry key at HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon. When examining the permissions in regedit, the Administrators group was missing. Only SYSTEM had full control and Authenticated Users had read access.
I really don't know how these sorts of things escape the beta testing phase. Did nobody with a virgin XP SP2 Dell image upgrade to SP3? Is my Dell something special? Seriously… What the heck?