I started having difficulty after an upgrade today (v1.7.0). It launched fine the first time after the upgrade and crashed on launch on all subsequent launches.
Thanks for your Kindleboards link. I removed the 3 PRC files in my My Kindle folder and now K4PCv1.7.0 launches fine. It seems the new version has some difficulty dealing with earlier versions of the MOBI format.
Some users may find old data versions lurking behind the newer MOBI extension so removing files with a MOBI extension may be required.