Password Text box getting cleared on selecting dropdown

[crythirty]

I have problem with password text box control. I have username textbox, password textbox, retypepassword textbox. And i have drowpdownlist with items Website, Newspaper, Others. After filling username, password, retype password in textbox. Whenever i am selecting items Newspaper and Others items from drowdownlist, password and retypepassword textbox value getting cleared. I have set in autopostback=true in dropdownlist control. Pls somebody help me where is my mistake??Thanks in advance