AFAIK overnet uses one part and one met file-extention as long as the file isn't complete (they're numbered something like 021.part and 021.part.met) just check out witch one it is by the file-size.¨
bopy the file to an other location and change the ending to the original one (avi or mpg or whatever) and try to open it.
some filetypes work like that and will bring just about nothing in the missing parts (i think overnet uses 9mb chunks for transfering if your not in flock-mode). other filetypes don't work and just bring up a error. don't know which one's though.
probably virtualdub might help on videos.
but i would suggest having more patience...
EDIT hey, that is my 1337th post :Q :beer: