Hi,
What type of .avi (or codec) are you using?
I think PI has a problem with DV footage. You make have to first change it to another codec or maybe even uncompressed before you insert it.
PS: I usually render my particles as a image file sequence with alpha and add them to my original video in my NLE. This not only keeps my video footage clean & crisp, it also lets me add my particles in layers so I’m not forced into making them look perfect in PI (and my cpu wont melt as I process them all at once).