Not sure if you have managed to resolve this in the last week but...
I assume you are referring to the folder rather than a file called .VirtualBox, this is a folder required by the virtualbox software.
In order to rename it you will need to use command line.
based on your post above i understand the folder is currently called 9.VirtualBox so to fix this you would run the following command.
1. Open a command prompt (Click Start >> Type CMD >> press Enter or click CMD.exe in the search results)
2. Copy / Paste the below line into the command window and press enter
Does that help?
move c:\Users\Peter\9.VirtualBox c:\Users\Peter\.VirtualBox