Oblivion

[jholland1964]

Give the exact full wording of the error.

[TurcoLoco]

Give the exact full wording of the error.

Also, assuming this is not a copied or download game nor you are using any fixes (trainer execution, dll hacks or No-CD exes), I'd uninstall the game (save your profile and saved games just in case but most newer games will prompt you during the uninstall process if you wanted to do so).
Move your saved game data to a location other than where it is in by default.
Reboot the computer, then remove the game folder under 'Program Files' folder. Afterwards, re-install game. If there were any updates (patches) for the game, first see if the default installation ran fine or not before installing the patches.