Microsoft is encouraging students to "steal" Office for $59.95. Frugal users of the free demo versions may be able to get through the semester before reaching the 25 use limit:
Eligible students may have free access to Microsoft® Office Ultimate 2007 Trial for a limited amount of time. Each trial provides (1) 25 application launches (each launch of an individual Office Ultimate application is counted as one launch) before the software goes into reduced functionality mode (at which time your software behaves similarly to a viewer, you cannot save modifications to documents or create any new documents, and additional functionality might be reduced)...
A free alternative is Google Docs, and with a recently added presentation tool, you have the triumvirate of word processing, spreadsheets, and presentations anywhere you are online and have a web browser. If you aren't doing anything too fancy, you should be able to load your Microsoft Office files into Google Docs. It's also easy to share all of the above with other users. And, it's free.
Another free alternative (if you are comfortable with installing open source software) is OpenOffice. A plus for OpenOffice is it's multiplatform.
More open source software: for Macs, for Windows.