Excel 2003 trial version? Excel 97 or newer should be able to open it,
except that if the spreadsheet contains features that are new to Excel
2003, then those features will not be supported if the spreadsheet is
opened in an older Excel version. OpenOffice should be able to open the
spreadsheet too.
Excel 2007 trial version? If the spreadsheet is saved in the newer
Excel 2007 .xlsx format, then you need Excel 2007, or Excel
2000/2002/2003 with the Office 2007 compatibility pack installed. As
far as I know, OpenOffice cannot open .xlsx files. If the spreadsheet
is saved in the older Excel 97-2003 .xls format, then you can probably
use Excel 97 or newer or OpenOffice.