If you insist on downloading and installing operating system upgrades yourself, Please, Please, Please, clean up your file system BEFORE you do. You can do this by running Disk Utility.
If the “Verify Disk” function says anything but “your disk appears to be OK”…Don’t install the upgrade!!! If the “Repair disk permissions” function takes more than a few minutes, and/or does not complete successfully…Don’t install the upgrade!!! Either of these symptoms indicate a problem that most of you are not equiped to fix, and trying to do so, or hoping the problem will fix itself will result in a more expensive repair.