Hi all, I'm new as well for the same reason as kal.
However I've done a little experimenting and discovered;
1. MP4 has nothing to do with it, it can be any video extension. In my case the problem occurred with MKV files.
2. It's dependent on what program I am using to initially view the video.
3. For example, if I right click the file and choose open with Media Player Classic Home Cinema, then I get the same as the rest, the PSP install program starts running. But, if I perform the same action and choose Windows Media Player or Nero Show Time the file opens fine and the PSP installer does NOT run.
4. Media Player Classic Home Cinema is my default program for MKV files, so I wonder if that is significant. BTW, it doesn't matter if I right click or double click the file, as long as it's Media Player Classic Home Cinema, I get the joy of watching PSP fail to try and install again, although it is, in fact, nicely installed already.
At least till I get fed up with this. I mean, I ended up with 2 patches within minutes of installing the program using a fresh download. The means either the program is buggy or somebody is sloppy with their updates to the download page. Both are other than good.
I'll give a try of moving the default program and see if the problem moves with it.
EDIT: Update - I discovered change default program did not help, but reinstalling CCCP (the source of Media Player Classic Home Cinema) did. After the reinstall I no longer have troubles with any video file, MP4, AVI, MKV or any others I've tried. I still don't understand what double-clicking a video file has to do with triggering PSP installer, but at least in my case, the reinstall of the offending, i.e. Media Player Classic Home Cinema, did the trick. Something similar might work for others. Additionally, PSP seems to be working fine so I don't think I've broken anything there (fingers crossed).