WinUAE is a highly recommended Amiga emulator under Windows.
On Linux you can try
E-UAE.
For both those emulators you need a legal Amiga rom image, either from a real Amiga or from Amiga Forever.
Native AmigaOS on a PC? Hmm. Perhaps you're thinking of the
Amiga Research Operating System (AROS)? It's not really mature enough for real use yet, but it's getting there.
Hope that helps.
