In Word 2003 or earlier, select the text, and then:
1. to superscript the selected text, either press
Ctrl+Shift+=, or click on Format | Font | Font tab | check the
"Superscript" box | OK.
2. to subscript the selected the text, either press Ctrl+=,
or click on Format | Font | Font tab | check the "Subscript" box
| OK.