I need help :(

My requirment is to show departments in one ddl and show the corresponding
users in that depart. in 2nd ddl

1rst question: How can i attach sharepoint List all user departments.

2nd question: i try to do work with 2 ddls as said in the threads, but i
cannot use filters at values, couz i have to use my infopath 2007 forms in
work flow in sharepoint, what to do with this(filters)?

I am quite fed up, i am tring this to dine from 3 days. Please Help