I don't believe Windows supports creating and displays windows from services. Services are intended to be background processes with no UI interaction, and any windows you do create won't be displayed on the screen (in fact, I believe Windows uses a separate session to host any UI generated from services to prevent them interfering with the current user).
Instead, you should look at having your form opened separately (and therefore hosted on the current session) and use cross-process communication platforms like WCF, named pipes or even sockets to transmit data. You could also consider writing data to a file on the file system (implementing appropriate file-locking) and have the form read this file to process any UI operations.
WCF[
^]
Named Pipes[
^]