I'm using the TESIV:POSitive guide. It wants me to uninstall Oblivion (to get it back to it's vanilla state: "Those users that need to revert to the vanilla state but do not have all of the original files (BSAs and folders) backed up somewhere, should probably go back to the previous step and uninstall the game").
When I go to setup.exe though, it only will let me install the program - when it's already installed.
That not working, I tried to delete the entire C:Oblivion folder. That didn't work.
What do I do now?

I'm trying to use the directions in this Forum, but when I get to this step:
8.)Go into explorer find the C:\Program Files\Bethesda Softworks\Oblivion folder (if it is still there) and delete it.
It says "You need permission to perform this action", I hit try again and it just reverts back to the same panel: "You need permission to perform this action".
Note that I have Oblivion in C:\Oblivion (because I'm on Vista).
So, I'm stuck there too.

Does your account have admin rights?

I got some help with this issue over at the Nexus Forums and figured it out. Thanks anyhow.