http://www.mediafire.com/?gc95ao6apqpdyqp (tested link is safe, and the program once extracted from the zip will run perfectly on any version of Windows)
But what if you use a Mac? Well, there's a similar thing by the name of "Pinta" - It's a beautiful thing because it follows Paint.Net's GUI, and as such if you work on the Windows version, you can just as easily work on the Mac version straight away when you get back home from working on Windows Machines to use your Mac:
http://pinta-project.com/download.ashx (install needed, look below for it's "portable" version)
Additionally, there is the portable version of Pinta that I haven't tried yet, it may or may not contain the Mac run time files:
http://www.softpedia.com/get/PORTABLE-SOFTWARE/Multimedia/Graphics/Portable-Pinta.shtml