Go into the BIOS of the computer and check it's USB settings. Has your computer initially on bootup had wrong date and time? It would automatically set the current date and time from internet. Boot it up once with it unplugged from internet. It's likely your motherboard battery is losing BIOS settings due to needing replacement.
Once that's out of the way, if needed, reload the software for the Canon camera.
I put this in General Help because I really don;t know where the problem is. I have a homebuilt computer (Intel DP35DP motherboard with Intel Core 2 Duo CPU running Windows Vista) that has been working just fine for the past 7 years. I have been loading pictures from a Canon PowerShot SD1100 IS via cable to a USB port without problems this entire time. A couple of days ago I switched from Norton Security to Avast Anti-virus. From that point on when I plug in the camera, the computer buzzes 3 times and nothing happens. I tried various settings in Avast, including disabling all of the "shields" to no avail. I even uninstalled Avast and rebooted, but incredibly, the problem remained. The only other thing I had changed was turning on the Windows firewall, so I turned that off, still to no avail. I have tried different USB ports on the board. The ports do work, because I can plug in flash drives and they open up just fine, although they also trigger the 3 buzzes, which has never happened before now. I checked the Canon site to download fresh drivers, but they apparently use the default Windows Vista drivers. I even uninstalled a couple of the USB drivers in the Device Manager and rebooted to force a reset (I say a couple, because there were about 16 listed and I really didn't want to go futzing with all of them.) There are no messages in any of the event logs. If Avast has a log of its activity, I haven't found it. I have run out of things to check, and I haven't even isolated where the problem is. The fact that it started after installing Avast would seem to be a big clue, but the problem remained after uninstalling it, so was that just coincidence? Any ideas would be greatly appreciated.