There's a way to do it. There's plenty of freeware programs that allow for you to do this.
Here's one called "Sweet Little Piano" (
http://www.ronimusic.com/sweet_pi.htm )
Here's "Bome's Mouse Keyboard" (
http://www.bome.com/midi/keyboard/ )
Here's "Trollo." It's only in VST format though. (
http://www.axxeom.fsnet.co.uk/trollo.htm )
There's more, but those are some of the ones I usually come across the most. Just google around if you want to check out similiar programs. I have never actually tried any of these, but I don't see a reason why they won't work for you.
BTW, you can use the PC keyboard like this in FL Studio. Just click on the little icon that's called "Typing Keyboad to Piano Keyboard," it's right above the "Metronome" icon. Or do "Ctrl+T" to enable/disable it.