The answer is in your EULA.
The Student & Teacher's Edition is limited to use on a single computer, although you can move it to a different one. Check the section on License restrictions on this page http://microsoft.com/office/forstudents or in the End User License Agreement (EULA) in the Help file of your Office XP applications.
I have the 2002 student teacher edtion of office that I bought just before the 2003 edtion came out. Microsoft says the 2003 student teacher edtion can be installed on 3 computers in the house. Does the same apply to my 2002 edition?