In excel is there a way to change a cell so that when data is entered to it it becomes *******, like a password box??

You could try this. I don't think it is exactly what you want, but you could probably modify to your needs!

http://www.danielklann.com/excel/hiding_text_in_a_vba_inputbox.htm

I dont see anything really easy, even with an event macro.
You can use a white font on a white background cell but the user wont know how many chars he typed in.
If only numbers are entered you can use the cell format ;;;
You also can use something like a Wingdings font or anyone else not easy to read, or better, create your own font with only * as character for a,b,c...
Anyway, you have to hide the formula bar.

Try this format :
;;;"********"

Same problem as others : the chars are readable when typed in.