Here's a quick summary of all the usual candidates when this type of question gets asked:
Irfanview is a great picture-viewing and editing program but I suspect (for the 'creative' stuff that you're considering) it might be a bit too basic. (Click on the question from Almendros, below, for further information about Irfanview).
Paint.net has plenty of fans, as does Google's Picasa, but once again I feel that they might not be advanced enough for you.
Serif PhotoPlus SE has more options than any of the above and might well be worth a look:
http://www.serif.com/free-photo-editing-software/?MC=FSSPHOTOPLUS
(Serif's software is always superb anyway).
However the 'biggy', without a doubt, has to be GIMP. It seeks to do everything that the 'full' (mega-expensive) versions of Photoshop can do and, indeed, is used by many professional graphic designers, etc:
http://www.gimp.org/
However, because it's a 'Photoshop standard' program, it's necessarily got quite a steep learning curve. (Some colleges offer year-long full-time courses in Photoshop because there's so much to learn!). So I STRONGLY recommend downloading the USER MANUAL as well as the program!!!
Have fun!