Just starting out with digital art

I've recently gotten a Surface Pro tablet with pen and am very impressed with its responsiveness and sensitivity. A long-time dabbler with pencils, inks and acrylics, I'd really like to explore digital media more.

What software do you guys prefer? I'm very well-acquainted with the Corel suite, but my version is a few years old (X5) and can only be installed via CD, and I don't have access to an external drive at the mo'.

Just looking for a decent, not-too-expensive program that would allow me to explore a bit more than the standard Windows sketchpad app does. Thanks!