What software are you using to make these? How can I change the start up video now? can I just replace the file?
These are done in sony vegas. For installing them, you have two options: 1. Backup the old file and replace it (under /media) 2. Put the new file in the same directory, and alter the text file to point at it. You can have as many or as few videos as you like, so maybe people could mix and match if some want in-game footage and others don't.
