Hello Ram,
You can use conditional formatting -> I am using infopath 2007 so
hopefully 2003 is similar if you are using that.. here are the steps
in 2007
1)Right Click on the text box
2)select Conditional formatting
3)Add
4)under "if this condition is true" select "users current role" is
equal to and then select the role from the right hand drop down
5) under "then apply this formatting" click the read only check box.

Hope this helps.