how can I export a copy of my imessages to my pc

I use an application on my Mac called PhoneView. It lets me archive my text messages on my computer as either .txt or .pdf files. There's a similar application, TouchCopy, that has versions for both Mac and Windows.

You can't directly, but you could either take a screenshot and send that to your PC or copy and paste messages into something like a note or word document etc, and send that to the PC.