Network Programming Problem

I am writing a program similar to the mIRC. First, my program connect to the server in LAN via port 9999. When users click on the list of nicks, a private chat form will appear. However, when I activate the private chat form by the statement [b]"privatechatform.Show()"[/b], the private chat form is crashed and my program too. If I change the statement into [b]"privatechatform.ShowDialog"[/b], it will be ok, but I cannot have private chats with multi-users. How can I solve the problem??? or anyone can give me the source code of the program like that.