Hi Ann,
This error is not with BIWS or overload in excel. You need to follow few point when you work on dashboard or exporting it to launch pad.
1. Dashboard -> files -> Preference-> Excel Options -> (Disable live office compatibility)
By doing this u can run excel even if u r exporting it( but better avoid using separate excel file when you using dashboard it creates problems in 4.0 sp5).
2. When you gets that error Go to Task manager and kill excel file that are running.
Thanks
Vijay Muniraj