Notepad++ is an excellent/advanced text editor and can be customized to your needs in an easy way. But how you “convert” it to a AVR custom IDE? Check what Vasilis writes on his blog vpapanik.blogspot.gr:
Now that you’ve chosen to drop Arduino and write in pure AVR-GCC for your next project, there are more IDE options to suit your needs. From the nice Programmer’s Notepad, included in the WinAVR installation, to the official supercharged AVRStudio. And the list is endless…
For the last couple of years, my personal preference for an advanced text editor, is, by far, the excellent Notepad++. I am a big fan of its customizable color themes and syntax highlighting, lots of editing options (like clickable links) and plugin support. Notepad++ is of course completely free and open source.
Using WinAVR with Notepad++ – [Link]