As Ryan stated, the TXT file format doesn't retain formatting, unlike Microsoft Word, for example.
However, I suspect that you actually want to dump the actual information about fonts into a text file. If that guess is correct, the approach depends on whether you are looking for a generic data "dump" of some or all of the fonts available on your system (or, at least, in Excel), or whether you have a need to retain information about specific cells in a specific worksheet.
Can you tell us more about what you are trying to accomplish? A sample of the actual expected text would be very helpful!