Ok, i guess i just wasted 30€...
AVI is a file container—not a compression format. The normal work flow is to determine what codecs were used to create your source AVI files and to then install those QT audio and/or video codecs if available. For instance, QT does not support AC3 audio natively. If your files have AC3 audio, unsupported MPEG1 layer 3 audio (I assume this is what you are referring to by MPEG3), "muxed" audio/video content, then you might be better off trying the free Windows version of MPEG Streamclip here. It accesses the QT components that are used by the QT player but also provides its own AC3 and support for multiplexed A/V content when converting it to common QT compression formats—including those used by itunes and mobile devices.
Also be advised that MOV files are generic in nature and can hold any valid A/V compressed data that is compatible with your system's current component configuration. This does not mean that all MOV files will be playback compatible with mobile devices. (I.e., it is the compression formats contained in the file container that make files compatible or incompatible with your iPad or iPhone.)
As to wasting your money, that depends on what you plan to do when editing files. MPEG Streamclip is great when working with working with AC3, muxed content, performing physical crops, and such, but it does not handle layering, masking, blending, scaling/offsetting video by individual track, or similar editing operations which QT 7 Pro can do. If you imaginative, my recommendation would be to hold on to the QT 7 Pro and use it for the things that only it can do. Also keep MPEG Streamclip (free) installed to handle the things it can do that QT 7 Pro can't. And, of course, there will be many things that either can do equally well.
