Are you referring to the Equation Editor? It is included with every version
of Word/Office but is not installed by default. To install it, go to Control
Panel | Add/Remove Programs, select Microsoft Office, and click the Change
button. In the first page, select "Add or Remove Features." On the next
page, expand "Office Tools." Select Equation Editor and set it to "Run from
My Computer." Click the Update button, and insert the Office CD when
prompted.