I have one license for Office 2003 Professional and one for Office 2003 Student and Teacher. You can install Office 2003 Pro on 2 computers (one being laptop). You can install Office 2003 S&T on 3 computers. I have plenty of licenses but I simply prefer the ribbon interface of Office 2007 and newer. The old menu interface for Office 2003 and older doesn't cut it for me anymore.
I have used the free OpenOffice, LibreOffice, and FreeOffice. What the current versions have in common is that they all support their own formats (.odt, .ods, and .odp) as well as the Microsoft Office formats (.docx, .xlsx, and .pptx). That makes it easy to move between them and Microsoft Office.
Another thing that OpenOffice, LibreOffice, and FreeOffice have in common is that you can switch between the old menu style of Office 2003 or the new ribbon style of Office 2007 at any time.
I should emphasize that the free versions of office and Microsoft Office may have incompatibilities for some lesser used functions. If you stick with the basic usage there shouldn't be a problem. Of course Office 2003 and Office 2024 are not totally compatible either.
Note Office 2003 and older use the old formats (.doc, .xls, and .ppt). If you have Office 2000, XP, or 2003 you should download and install the Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint. That allows you to read/write the Office 2007 and newer formats (.docx, .xlsx, and .pptx) also.