Is is possible to download Powerpoint 2003 (not the Viewer).
If you mean "Can I download a free working copy of PowerPoint 2003" then no.
At one time you could get a time-limited demo version, but now that 2007 is
out, that may no longer be available.
Or, is
Powerpoint 2007 compatible with Office XP 2003?
Office XP = Office 2002
Office 2003 is a different product
There are converters you can install into either that enable them to read/play
PPT 2007 files, or you can save from 2007 to the earlier format.
Does this give you total compatibility? No. The earlier versions don't
inherit the features of the newer version.
