What version of office you have on your xp machine?
may be because your powerpoint file is in the newer pptx. You need to download the compatibility pack to open it on office2000:http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=3
If the file haven't already associated with the program. Right click on the powerpoint file>open with>choose default program
select office2000>check "always use the selected program to open this kind of file">ok