Request that the person who created/owns the 2003 spreadsheet save it out
using Save As and choosing the "Excel 97-2003 & 95/Excel 5" option.  It will
make for a larger file, but will be usable by any current version of Excel,
including 2002 from Office XP.

The versions of Excel 97 through 2003 all use the same internal file format,
so files may be freely created/shared/opened/saved between versions. Of
course, if you use a tool in XL2003 that didn't exist in 2002, you won't
have it but you shouldn't have problems.