C#, VS2008, Thread, Progress-Bar within form.

(a) I have ordinary window form called AccessForm with button and a ProgressBar
(b) When the button is clicked, it will performs an operation involving internet and files, which is slow process,
(c) I found that while the an operation is ongoing, the AccessForm is hanged waiting for completion of the operation.
(d) This affects the ProgressBar because it does not show progress even when I made a value change.
(c) Thus I'm using WorkerThread that encapsulate the slow operation in order to keep AccessForm active and available for user,  so user can stop the operation when needed.

Question
(a) What the best way to update the ProgressBar within the AccessForm when  threaded routine can completed one stage of several tasks.
(b) I prefer not to send code, however is there similar example?

Thanks.