I have a few video converters but my main one, the one that gets the job done with no muss or fuss is the beloved (and free) MPEG Streamclip by Squared5. Be ye a Mac users or Windows user it is hard to go wrong with software.
Before installation you do need to have the free QuickTime player installed on your system for the program to function properly.
If you have a camcorder that records in the .mod format and your video editing program does not accept the .mod videos then you need MPEG Streamclip to ease your mind. Convert those suckers in an .avi video and you are good to go.
Here is a quick tutorial by Dad200UK2000 showing how to convert an .mp4 into .avi
In order to reduce the possibility of spy or trojanware always download from the primary web site or a trusted place like CNET Downloads or Major Geeks.