Thanks Ed.
Actually, I used AVIsynth for Windows. It's a weird scripting language for video which allows all sorts of strange things. Once you got your script right, you just drag & drop the text file on VirtualDUB which calls the Synth engine to do the hard work in the background. It's not a windows default and sadly, the program doesn't even have a user interface... But I also found (after sweating this one out) a program called AVE Visual Editor. It makes scripts for AVIsynth... but they are often buggy and you have to re-do things.