Method 1.
Sorry that you can't download stuff. In MS Paint, create a new drawing. In the menus, choose "Image, Attributes.." and set the size to something small. Say, 16 pixels wide by 32 pixels tall. Now, use "View, Zoom, Custom, 800%." to zoom in. Use the pencil to draw in individual blocks. When you're done, you can go back to 100% zoom to see it pixel by pixel on your computer monitor. If you want to make it bigger, you can do Image, Resize/Skew.. and change both the horizontal to 200 or 400. Unfortunately I think this anti-aliases and you may loose some blockyness.
Method 2.
Do the same as method one, except make a bigger picture. Keep it at 100% zoom. And use the paintbrush and choose one of the big block brushes. Now you'll be able to make a bigger blockier look, but you'll have to ensure by yourself you'll stay on your own grid.