If you don't want any maps to be lost, put your Exports and Terrains folder into another folder where you'll remember you put them. We will need them soon.
Go to Local Disk (C:) >> Feral Heart and there should be an un installer that looks like this:
(http://i41BannedImageSite/ohtrgw.png)
Click it and un install FH from there and delete the FH folder then come back when you're done.
You're back? ok.
Re-install FH here (http://feral-heart.com/index.php?option=com_content&view=article&id=23&Itemid=40) and once you've done that check to see if it works.
Remember the files you put into a different folder? Good. Place the Exports file into Local Disk (C:) >> Feral Heart then once you have done that, replace it. After that go get the terrains folder and place that into Local Disk (C:) >> Feral Heart >> Media and the replace it.
If you're still having trouble, try looking here: [Linky link] (http://www.feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=10423.0)