With the Office Compatability Pack installed, Office 2000-2003 can open and edit Office 2007 and later files. Features introduced with Office 2007, though (eg content controls, Excel rows beyond 65536) can't be used..docx - Word 2007-2010
.xlsx - Excel 2007-2010
.doc - Word 97 onwards
.xls - Excel 97 onwards
If you are running an earlier version of the office system, I would recommend a few solutions:
2. Click here to make it forward compatible. Note that this makes the file readable. I doubt it will let you modify it though.
3. Save it as a .doc file instead from the source, or after you have made your computer forward compatible as mentioned just before.
4. Install OpenOffice. It supports it as a free Office Suite alternative. Feel free to flick the files back and forth as you choose.