[i0xIllusi0n]

You can't listen for a key system-wide or "globally" by invoking anything. Invoking has nothing to do with awaiting events. Also the code posted on that SO link, by the OP is terrible. The suggestions are the proper way if you read on. Just should be worth mentioning I thought. SendKeys to invoke the print screen key, is a very stupid/silly and lazy way of doing it.

Ahh, but you can. I've made programs that listen to keys on a global scale, not just when your program is the currently active window.
And if you haven't already noticed, I said I have never tried what I linked. All I did was Google it, just like anybody else can. I don't care what I found. As long as it was on-topic.