Q. I have a mysterious folder called config.msi that now holds nearly 78 megabytes of files. These files all end with the .rbf extension. What are they, and can the contents of this folder be safely deleted?
A. Microsoft's developers, in their infinite wisdom and generosity, created this folder and these files on your system to allow you to back out of an upgrade to Office 2000. In the event that you experience problems with your installation, these files allow you to uninstall Office 2000 and go back to your previous version of Office.
If you are confident that your Office program is working properly, you can delete this folder and all the contents.
I would suggest that you may just want to rename the folder to config.msi.old and see how things go for a week or so. If all is well, go in and reclaim that disk space.