Microsoft Office alternative for Windows?

Try OpenOffice.org

Edit: Since the creation of this post, many if not most of the supporters of OpenOffice.org have migrated to LibreOffice, which was originally based on OpenOffice.org and now has a more active development than its parent project.

There are a number of web-based Office services that are also free and offer some of the functionality of Microsoft Office, including:

Office Web Apps

Google Docs

Zoho Office

While these tend to lack the more advanced features of client-side Office suites, they do have excellent collaboration features.

Search for your school through the MSDN Academic Alliance. Even if it's not free, it will be significantly cheaper (to the tune of $30 or less).