Try the following:
1. Highlight you text box.
2. Left click your text box.
3. Context menu appears. Select 'Format Text Box'
4. Go to 'Text box' tab.
5. Select 'centre' from the three options.
In W2003 and before, you cannot vertically center text within a text box.
You can use a table instead of a text box (or within the text box) to do what
you want.