When you uninstalled oblivion did you remember to delete the .ini file?
Exit Oblivion, then try renaming or deleting the oblivion.ini file (...\My Documents\My Games\Oblivion\Oblivion.ini)
Oblivion will then redetect hardware. It will also, probably, not recognize your video card and set everything to medium. Just change the settings according to what you think your card will support (You can always change them later)