SX40 video problem
If you've used this computer before for video from the SX40 and it worked OK before, I don't know why it wouldn't work this time, unless it just got corrupted when uploading it.
However, if this is the first time you've tried this computer, it may be that it is just too slow for the HD video from the SX40. My newer Win7 computer at home plays them just fine, but the older computers at work can't handle the HD stuff. They just jump around like stop motion photography.