for everybody, who wants top work with the old good pixel aspect ratios (pal widescreen - 1.42 or pal dv - 1.07) should read this thread.
For a discussion about if this modifikation is useful and correct, we should start another thread. Here is only, what you can do, if you want the old goood pixel aspects back in your settings.
This is NOT only the disable pixel aspect correction, that is explained in some threads. This modification gives you selectable settings.
The only thing you have to do is to search for the "interpretation rules.txt" You find it in your installionpath.
For Windows: C:\ProgramFiles\Adobe\Adobe After Effects\supportFiles\
For Mac : it should be the same one.
Open the "interpretation rules.txt" and insert following lines
#Custom PAR The good old CS3
0,0,0,"0000",*=768/720/"D1/DV PAL (CS3)", *, *, *, *, *
0,0,0,"0000",*=64/45/"D1/DV PAL 16:9 (CS3)", *, *, *, *, *
0,0,0,"0000",*=648/720/"D1/DV NTSC (CS3)", *, *, *, *, *
0,0,0,"0000",*=6/5/"D1/DV NTSC 16:9 (CS3)", *, *, *, *, *
I placed the 5 lines under the first 2 parts of NTSC and PAl Rules. But I am sure you can also write it down at the end. The Line beginnig with # is only a comment line. After Effects gives you now 4 new selectable pixel aspects in your composition settings - like in this picture:
Be careful if you are a windows user (win7 or vista) you have to manage the right settings of the file to be able to save your changes.
Hope it is helpful for you. Enjoy.