'Retarded' may be a bit harsh - perhaps 'slow' might be more appropriate.
You're assuming that performing innocuous read only file operations is sufficient cause to flag the macro as being a virus.
Consider, for example, a legitimate macro which would present the user with a list of monthly sales reports. I haven't done spreadsheets since running Lotus 1-2-3 on a VAX mini computer, but your macro would essentially end up searching for 'SALES*.DOC' files - almost exactly what this one is doing.
Would you bar any such operations? If so, you run the very real risk of having so many false positives that it essentially becomes useless to scan macros; simply block them by default on Office's side.