If you have just upgraded InfoPath to 2013 and when you try to open InfoPath forms you get the following error string in your error dialog box details:
Exception from HRESULT 0x80070002
it's probably an InfoPath cache issue. I upgraded from Office 2013 Preview to 2013 Production today and noticed that some forms were opening fine but others were not.
The solution? Run "infopath.exe /cache clearall" command. That solved my problem beautifully.