I have been busy.
If you type 127.0.0.1 into your browser's address bar, do you get anything or just an error message?
If you installed XAMPP on your main hard drive, disabled your firewall software from interfering with communications (including the built-in Windows Firewall), and allowed XAMPP to run as administrator, it should work unless you made changes to the configuration files. The only other possibility I can think of is that there is another programming listening on port 80 and that is preventing Apache from binding on that port and causing it to shutdown immediately.
Let's look at what programs are listening on your ports and see if anything is listening on port 80. Go to your command line program. In Windows, you can go to Start Menu and in the search box type "cmd". Or you can go to Start Menu -> All Programs -> Accessories -> Command Prompt. When you get your Command Prompt opened, type this command in it:
That will list all active programs listening for connections and the ports they are listening on. Near the top of the output, look for something like this in the Local Address column:
If you see that line with the State column showing LISTENING, you already have a program listening on that port. If you do not see it, that is good because that cannot be the cause of your problem.
Other than a program already listening on port 80, I am out of ideas. I think that when XAMPP is setup you can choose to install Apache as a service. If you did that (and I do not know if you did), it would mean that Apache is already running as a service. If Apache is already installed as a service, it would show in the Services section of the Administrative Tools area of the Control Panel.
I am out of ideas. If that doesn't help, then try downloading XAMPP again and reinstalling to another folder on your main hard drive.