The correct one is that installed by ATI's drivers. The trick is making sure that gets installed and no other.

Don't trust drivers bundled with Windows or from Windows Update.

It does not appear that the ATI Catalyst 4.5 drivers for WindowsXP actually installs an Opengl32.dll file. I tried installing the drivers several times, and even deleted the exising Opengl32 file before the last re-install, but Catalyst did not replace it.

When I put back the original one, OpenGL seems to work with programs such as Treemark, and GLinfo identifies my OpenGL as version 1.5.4332 WinXP Release.

I'm not exactly sure what the Opengl32.dll does, but since I briefly had installed a version that allowed proper operation (before Windows killed it), I have to assume that this is where my problem lies.

Any insights?

Nonono, don't delete Windows XP's opengl32.dll, it's essential for proper GL-support.
ATI installs the file atiogl32.dll (or the like) in your system32-folder; it's only an icd, not a full opengl-implementation.