I have a feeling I'll be the only one who's had this problem but eh, figured it was worth trying anyway.
Basically, the SC and 3rd installers install a bit of the game then... freeze, the drive powers down, and nothing further happens. I can't fully close the installer without shutting the PC down. Origin, Felghana, and Sora FC all installed fine, but SC and 3rd... won't.
As for what I've tried... - Tried my legal SC disc (Vista version, on my Vista laptop). First error. - Wondered if it was a problem with the disc itself, so tried downloading it and running it from a virtual drive. Same thing. - Tried my, again legal, 3rd disc, same error. - Wondered if it was Vista, tried my SC disc on a friend's XP laptop. The drive didn't power down, but it jammed up anyway.
The strange thing about mine is when it powers down I'm able to switch from the installer window, and if I move around over the icons and files on the disc in Explorer it sometimes starts up again and resumes installing. I can't quite get my head around why it's doing this, especially as it's on both PCs - clearly it seems to work for most everyone else. I finally got it to install by copying the files manually into a folder on my HD, so it doesn't seem to be a problem with the files themselves either.