If the problem is in the header of the file you can try to take a look at the file in binary mode (for example by using HexWorkshop or similar applications)
Here are the information you need to compare:
http://www.fastgraph.com/help/avi_header_format.html
Probably you'll have a hard time to track back the problem since some fields can't be simply recognized, but if you'll find the problem in the simplest ones (for example the last 6 items), you can fix the avi header by yourself :roll:
Maybe the problem is in the Video window size (the starting problem of this thread). The player read the video size from the file header (wrong one), then you change aspect ration and once you click Original it reads the values from the P frames images instead of the header, resulting in a correct aspect ratio..
I'm only supposing this, but if the problem really lays there... you only need to binary enter two (probably only one in your case) 4 bytes values and the avi is fixed :D