Had a few spare minutes so I made this little tool which attempts to retrieve the product key from registry (I'll post it on the steam forum too once they finally approve me...).

Disclaimer: might or might not work, at worse it does nothing ;)

Download the tool (20kb), extract and run it.

Check the almighty 30 lines of code behind it. (compile with NSIS)

The original thread on steam that explains how to retrieve the key.

Hope this helps some