You don't need a sophisticated digital painting program to create an effective Christmas card suitable for emailing or printing, and it doesn't take long to make. Basically all you need to do is scan or photograph a drawing (or do one digitally) that has a strong, dark outline, then drop in the paint colors.
Most photo editing/paint programs have a "fill" option, for filling an area with a color (usually an icon like a bucket tipping over.) Ensure individual areas (e.g. the star on the tree shown here) are completed closed so the color doesn't spill into other areas when you fill an area. Color in, sign, and email.
Free Painting Software for Windows
and Free Photo Editors for Windows
(From About.com's Graphics Software Guide)