:wtf::omg::confused: X| Have you read the articles I provided links for in my previous post? If you have, you did not understand any of it. DefWindowProc() is probably the absolute worst place you can put a call to ::ReadFile() in since it processes unhandled messages and may get called frequently or not at all. Have a look here: http://msdn2.microsoft.com/en-us/library/ms633569.aspx[^] for further information about window procedures. You should also have a look at http://www.codeproject.com/system/#Hardware+programming[^] since you'll find some different approaches to writing serial communication. "It's supposed to be hard, otherwise anybody could do it!" - selfquote