Converting a prezi into a video is possible via screencasting. There are a lot of different screen recording software packages available:
http://en.wikipedia.org/wiki/Comparison_of_screencasting_software#Comparison_by_feature
From personal experience we can recommend www.techsmith.com/camtasia.asp (paid; free trial), but any other should do the job as well. On a Macintosh, the built-in QuickTime tool can record your screen as well.
Either way, you will probably be able to choose to record the entire screen, or just a pre-defined area. Adjust the prezi's windows accordingly (to hide the control buttons for example). Then, just record while clicking (and talking, if the software supports simultaneous microphone/audio recording) through your prezi at the desired speed.
