If you're using an old version of Microsoft Office (2003 for example) you will experience problems if someone sends you a spreadsheet or document in the new format (usually ends with an x on the end of the file name). If this is the case you can download a copy of Open Office for free which will enable you to open these going forward.
It's a good piece of software and is free compared to the many pounds you can spend on Microsoft products. Depending what you need, it's well worth a look, especially if you've got children who need an affordable solution for their project work. The other bonus is that you can save documents you create as if they are Word or Excel documents too - so there are no compatibility issues if you're using in more than one place.