Working between Keynote and Powerpoint
Powerpoint to Keynote
If you have a Powerpoint presentation that you would like to modify in Keynote, most .ppt files can be opened directly with Keynote, automatically converting them in the process. You can do this on a Macintosh computer by choosing Keynote from the "Open With..." menu on a Powerpoint file, or by selecting a Powerpoint file from the File > Open... dialog.
Keynote to Powerpoint
If you have a Keynote presentation that you need to run on a Windows computer, this can also be easily done. You can simply export the file as a .ppt (Powerpoint Presentation) from the File menu and choosing the Export button.