A surprise came when I installed the next round of Microsoft updates (from our local WSUS server). An "Junk mail filter update for Outlook 2003" (KB931764) was offered to me, although I no longer had Outlook 2003. Not knowing what this means, I installed the update.
Right after that had an error in Outlook: "The File ExSec32.dll is incompatible with Microsoft Office Outlook. Install Outlook Again.", when opening any Outlook item. Apparently the "Junk mail filter update" reinstalled some of Outlook 2003 DLLs that were not compatible with Outlook 2007 (I guess this was the reason why they could not coexist in the first place).
I removed ExSec32.dll in Office11 folder, then get the same error about RTFHTML.dll. Removed it as well. Outlook items started to open, but after a few more errors, I resorted to reinstalling Outlook 2007.
Ever since then, each monthly update of Junk Mail Filter for Outlook 2003 was offered to me by Windows Updates, and I had to manually deselect it. Finally, when I uninstalled Office 2003, the problem stopped.