Just bumping an old thread here. I had to do a fresh install on my garage computer after a power outage crapped my windows installation beyond repair. When installing Windows updates, antivir and other necessities, I remembered that Nefmoto needs .NET Framework 3.5 to work. I installed this manually before I installed Nefmoto. Everything worked before just fine with the same setup but after the fresh install, Nefmoto wouldn't start. Just opened the process for a few seconds and shut down without the main window. My fix was to uninstall .NET and Nefmoto, reboot, install Nefmoto and launch the .NET install from Nefmoto installer.

Maybe it was a localization or some kind of .NET framework version issue, I'm using Finnish WinXP. Worth a try if You're having similar problems.