http://www.pcfreetime.com/ This is one of the easier converters I use(Gonzoe and Jbird found this) that converts any file.Download this and then click the alltoavi button on the left.Add file the mkv or whatever else you pick up on the net thats not avi format and click open when you find it on your desktop.Then go to the output setting button and click High Quality and size.Before you leave that page go to videosize and open it and click 720x480 (this will make good quality but not be huge file).Click ok and then ok again.Click output folder and pick which folder on your desk you want it at.Click start.