I use Paint:

1)when I'm bored. http://www.flickr.com/photos/bhu1st/4911339994/

2)when I've to crop/edit quick screenshots

3) while teaching students about web design process (I do design/prototype quick web page layouts using paint).

4)meme generator ! Stitch images quickly to share it online.

While switching to Linux or other OS I sometime unknowingly try to hit win-r->mspaint.