How do I repair this error in QGIS:

The error message was received whilst trying to run v.kernel from the modules list. I can however run v.kernel from the GRASS shell.
A while ago, I installed a new plugin which apparently moved or deleted a whole lot of files. The result was that I couldn't use NVIZ. Uninstalling and re-installing QGIS didn't help so I finally resorted to manually locating the files and replacing them. Not sure where to find this file however. Is there anyone who can assist me how to repair this?