Any particular reason you need .TGA? .IFF is just as good and faster to render and works in After Effects. The reason .iff is coming up as Microsoft Picture It! files is because the file format is associated with that program. You'll have to go into your system settings and change the file associations. Or just right click on the file, choose OPEN WITH... and choose the program to open them with and check the box that says ALWAYS OPEN WITH THIS PROGRAM.
Dave Baer
Professor of Digital Arts
Digital Media Arts College
Boca Raton, Florida
dbaer@dmac.edu