How do I vertically center text in a normal text box?
A quick search of the SDK seems to indicate that you want to browse the section on "Edit Controls" under "Edit Control Text Operations". There are dozens of styles and messages, and your answer is probably in there.
After a bit of reflection, I guess I'd want to know more about this textbox you're using...
are you setting its height and font? What is the specific situation you're trying to handle?
-John