Converting CSV to fixed-width in Notepad++?

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.