Hi Gary,
You probably can't. This is the margins set by your
printer, not PowerPoint.
Try clicking on Start, Settings, Control Panel, Printers,
Right click on your printer and select properties. Check
the margin settings and see if you can change them.
You'll always have some white border because the printer
isn't going to print all the way to edge of the paper but
you may be able to make it smaller.
HTH,
Ms G