In Excel, under the Data tab, there is button called Text to Columns. The allows you to define the delimiters (tab, semicolon, comma, space, etc) and preview the effects before implementing. I also think that the next time you open a "text" file, it will use the same delimiter setup as previously used, so that's probably your issue.
If you right click the file in Windows Explorer, and go to Open with... you can select a different program, and even make it permanent. If you have Notepad++ installed, it should even list Edit with Notepad++ right in the drop down menu already.