Some great resources in this article on Microsoft.com, including:
the fantastic, FREE Graphing Calculator software in Microsoft Student 2006
the Equation Writer in the Education Pack for Windows XP Tablet PC Edition
and nifty Excel charting tips and templates
Oh, and if your student, like mine, is crazy for Sudoku, try this and this. If you have Vista - there is a Windows Live Sudoku gadget, and a Sudoku Assistant.