for recommendations have a look at these:
Adobe Premiere Elements 7
A very efficient and powerful editing suite that allows multiple audio, multiple video tracks (and therefore layering with differing opacity), Picture in Picture, green-screening, many transitions and effects, ability to burn straight to DVD from within the software, post to Youtube from within the software and much more.
Vegas Movie Studio Platinum Edition 9
again multiple tracks for audio and video (only 4 of each - compared to Adobe's 99), Picture in Picture, green-screening, many transitions and effects
Both accept input from a range of file formats and output a similar range, both are relatively easy to use but offer powerful editing capabilities.
I prefer Adobe - mainly because I started on Adobe and have used Photoshop for years so am familiar with Adobe layouts and terms.
Download a trial version and try them out - best way to see which suits your way of working.
sorry I can't help you with Imagemixer - seems to be not very good software judging by the number of threads that deal with Imagemixer problems.
May just be you don't have enough available memory to render and save the file - though it is not overly big as video projects go. Check your page file settings and usage by using Control Alt Delete while trying to render the movie - click on the performance tab of the Task Manager and see what your CPU usage is and your Page File usage - if they are maxxed out then that is probably the problem.