OpenOffice is an open-source office suite, comparable to Microsoft Office that includes:

• Word Processing - similar to Microsoft Word
• Spreadsheets - similar to Microsoft Excel
• Presentations - similiar to Microsoft PowerPoint
• Graphics
• Databases

OpenOffice can easily read and edit your existing .doc or .xls and many other files without changing any of it's contents or structure and will allow you to continue editing using that same format or offer you to switch to it's native format.

Absolutely free of charge for any purpose

It's an excellent alternative to Microsoft Office especially that it's free to download and use:

http://www.openoffice.org/