• On Windows Vista, the UAC (User Account Control) protects the Program Files directory (usually c:\Program Files) against write by regular users. This can be a problem for Bash and other mod editing and management tools (TESCS, TES4View, OBMM).
• Best Solution: Install Oblivion to somewhere other than under the Program Files directory. (E.g, install to c:\Games\Oblivion). Such an alternate directory is not subject to UAC control and won't have the same access and virtualization problems that the default install will have.
• Alternate Solutions: Alternatively, you might always run as admin, or turn off UAC – but both of those approaches are ill-advised. For more advice, see the Unofficial Help page at UESP.