Almost every Windows-based computer comes with Paint pre-installed as one of the accessories, along with Notepad, games and the calculator. This drawing tool will do fine if you're only interested in a few basic features, but for anyone who's going beyond a simple sketch, it's lacking some key components.
If you'd really like to create digital paintings, there's a better option out there. It's called TwistedBrush. This software was previously called Pixarra Sketchbook, but it still has many of the same features. Some people even use this program as an alternative to PhotoShop.
The free version of TwistedBrush offers many different paintbrushes and modifiers that you don't find in similar programs. These brushes let you create multiple effects such as oil paint, soft pastel, gel pen, pro pen, blender, razer, etc.
For each of these tools, you can also adjust the density, opacity and thickness to create an even wider range of effects. The oil painting option, for example, will smear the colors together when you overlap them on the screen, just like a real paintbrush would do on a canvas.
To edit an existing image, just open it in TwistedBrush by clicking: File >> Load From File As New Image (Import Image).
TwistedBrush can complete most of the tasks PhotoShop can, but it is a scaled down version. You can also use a digital pencil pad if you don't want to sketch with your mouse. To do this, you just need to plug in the pad's USB drive.