Originally Posted by Thrik
I'd actually recommend Paint.NET
. It allows you to do a lot of manipulations easily and quickly (text, blurring, glows, etc), but most importantly it's very user-friendly; you're much more likely to find it comfortable if you use Windows.
I use Photoshop, and Paint.Net has many of the same utilities and functions you'd see in a high end commercial product. GIMP is great, don't get me wrong, but when I did comparison tests, Paint.Net felt more natural based off of using both Windows and Photoshop.
Since they're both free though, give'em both a shot and see what's more comfortable for you