Can I fight against a PC player while I'm playing on Android?

Yes, you can. Or in other words: You don't have a choice.

Hearthstone doesn't provide several accounts for several devices or systems. You will have one Battle.net account to play with, which you can use on any supported device.

As your progress is stored into your Battle.net account, it doesn't matter where you progress - it is shared on any device.

You can proof this by creating a new Hearthstone deck on your PC, open the game on Android and also see your deck in here.

"I don't have a choice?"

You asked if you can play against Windows users. Yes, you can, but on the other side you can't choose from which region or device your enemy should come. This means you can't exclude PC users when playing on mobile device. All users use the same Battle.net servers linked with their Battle.net accounts and are thrown together.

So yes, you can battle your friend, no matter on what device he is active.

This is also stated in the Hearthstone wiki on gamepedia.com:

Hearthstone is playable cross-platform. Players can play against any opponent in their region regardless of the device or platform used by each. Basic gameplay is identical on all platforms, although there are numerous differences to the interface to address the needs of mobile and touch-screen devices. Other cosmetic differences include the use of a different starting screen (see right).