What is your Oblivion install folder?If it's the default(Program Files(x86))then Win7 and Vista have issues with the UAC.Create a Games folder in your C: directory and install Oblivion there.
I first tried making a new directory as I was aware of issues in Vista of allowing the game to install to the default directory. The new directory that I installed Oblivion to is C:\Games\Bathesda Softworks\Oblivion.
The TES4Gecko version I am using is 15.2 and I've installed it in it's own directory, in the Oblivion directory, on my desktop directory. regardless where I put it it cannot find the Oblivion install directory.
Just out of curiosity I did a clean install of Oblivion to the default drectory (Program Files(x86) and had the same results.