I have Mac OS X 10.5.4. I seem to have trouble downloading a trial version of Photoshop from the Adobe site.
First off all, when I download it, two icons come up on the desktop. One is in .dmg file and the other is .dmg.part file. When I try to open the .dmg file, it says the disk image failed to mount, so I tried opening it with Disk Utility. It says the file can not be recongised. So I tried opening the other icon (.dmg.part). Disk Utility opens, but other than that nothing at all occurs. It doesn't even come up on the Disk Utility.
I've tried to find a solution but so far nothing has worked...

Note that this happens to ALL downloads so I can't just download another software to fix the problem!

You're trying to open it before it's finished downloading. The .dmg.part is a temporary file that Safari uses when it's downloading a large file. Once the file is finished downloading, it removes the .part file and moves the data into the original .dmg file.

The .dmg.part file is a temporary file and indicates that the .dmg file is still downloading. The operating system handles the .dmg.part and deletes it when the download is done.

The .part temporary file is always created and deleted for any downloaded file.

If you try to open a .dmg file when the .dmg.part file still exists, the operating system will return an error because the .dmg file is still incompelete. Just wait 'till the .dmg.part file disappears, it means the downloading is finished.

There should be a progress bar on the *.gmg.part file, that will tell you how close it is to done if the Safari download window is closed