I suddenly was receiving the same message "Missing BASIC11N.MSI file"although I had done nothing new to the version of Excel that I have been using for several years on my computer. It said I was missing the Installer File for MS Office Basic Edition 2003 and needed the disk.
After searching the web I stumbled across someone who suggested that inside the "Office11" file folder set up a new file named "XLStart". I thought I would try it - IT WORKED! My Excel files start right up. It is an empty folder, nothing more than a name.
So here is how the entire path lookswith XLStart being the new file folder:
C:\Program Files\Microsoft Office\OFFICE11\XLStart