This works with later versions of LMS Test.Lab, so I assume 11B would behave the same. I believe you've changed your default program to open *.txt files with Excel. If I change the default program to open *.txt files to be excel, I see this behavior.
Right click on a file with the *.txt extension (or if your runlogging file has a different extension then select that file type) and select Open With. If you don't see Open With, try Shift-Right Click.
Select Open With, Choose default program.
Select Notpad, Wordpad, or whatever text editor you desire AND turn on the option "Always use the selected program to open this kind of file."
Test.Lab will now use this program - you don't even need to restart Test.Lab