Is there a way in Notepad++ to convert a comma-separated file to fixed width columns?
Use the built in TextFX menu.
TextFX -> TextFX Edit -> Line up multiple lines by (,)
I didn't discover this on my own, I found it on sourceforge while trying to figure out this exact question!
In the meantime I have just found a very nice tool for that. It's called CSVed and available here, with many many nice features. If you hate Excel when viewing CSV files like I do, you should give it a try.
Yes, it's possible in Notepad: menu Edit -> Replace... -> ReplaceAll ',' by '\t'.