First off a little m ore information.
Can you access websites other then this one your trying to download from? Can you download from those other websites? (Rule out a website issue)
Try flushing your DNS
go to START > RUN > Type "CMD" > Press Enter(On your keyboard) > type " ipconfig /flushdns" then type "ipconfig /registerdns"
That should refresh the DNS. Then what were going to do is a general 'Reset' on your Internet Exploder 8 (IE8). Launch Internet Explorer (Your web browser) and go to "Tools" In the menu bar. (By the top address bar normally) Select "Internet Options". This wil;l open the options for IE8. Select the "Advanced" tab. Its on the top by the other tabs. Under the Advanced tab select "Reset". A pop-up will ask if your sure and a summary of the reset. Check the box "Delete personal settings" then click "OK"
After it finishes it will state you need to close the browser for the settings to take affect. Go ahead and close the web browser.
Reopen IE8 and try to download something. If you get the same message, hold "Ctrl+Shift+Esc" on your keyboard (No do not hold the "+" thats just soemthing we use to indicate all 3 at the same time) This will open task manager. Another way to open task manager is to right click your task bar and select task manager.
once task manager is open, select the "Processes" tab. Sort the processes by name by selecting "Name" or "image Name". Look down in the "G" area for a program called "gamevance" or "gamevance32.exe" If you see this you have got some adware on your computer. Sadly this stops IE from working correctly. You can end the task int he processes and IE should start working correctly again. You will need to remove the adware from your system if you dont want to have to do this every tiem you try IE.
To remove gamevance open you "My computer or "Computer" from your start menu. Select your "C:" or "Local drive" Expand the "program files" folder. Find a folder int here relating to "gamevance" It may be hidden under some oddly named programs tryign to discuise itself as a video game.
try reinstalling IE8. http://www.microsoft.com/windows/internet-explorer/worldwide-sites.aspx
Of course you will have to use another computer to download the software if your current wont download.
If reinstalling doesnt solve the issue. Try another browser like firefox. personally Firefox is much better then IE8 and you can even download free ad-blocking software to make your pages load faster and store less ad cookies on your machine.
I am using Windows7 and IE8 on a Dell2350. The computer was working fine till last Friday and now I can't download anything. I keep getting a message saying "Unable to download... Unable to open this internet site. The requested site is either unavailable or cannot be found. Please try again later." The dialog box itself doesn't show how big the file is or the download speed or the time left. I also keep getting a message saying a program has corrutped my program setting.