The most likely cause is that something failed in the Windows Installer (which Eraser 6 uses) and your context menu registry entries were not created, or were malformed. Unfortunately, reinstalling on top of a failed install often only perpetuates the problem.
I suggest that you uninstall Eraser completely, delete the whole of the Eraser 6 folder in your (hidden) Userdata/Local folder, run a registry cleaner, then (if you feel confident enough to do so), use the registry editor to search for registry keys and values relating to Eraser, and delete everything that you are sure is specific to Eraser. This gives the best chance of a successful reinstall. If it still fails, do come back; Joel can help with the more esoteric issues related to Windows installer.