I'm afraid reinstalling the game to a different directory did not work. I believe my patches are up to date and I am not using any other mods.
Something I neglected to mention, though, is that when I close the Construction Set, I am given almost always given an error message telling me that there are still objects in the model loader. Would this be significant?
Nope. That error message is completely safe to ignore.
Have you verified that the .esp file you created with the Construction Set is in your ..Oblivion\Data folder, that it is showing up in the list of plugins in the Oblivion launcher, and that the box next to it is ticked (indicating it is activated)?
When you say "I believe my patches are up to date...", do you know for sure? Oblivion.exe shows as version 1.2.0.416 in Windows Explorer?
Have you tried using any other mods? Do
they work?