Where are you trying to download Firefox from? Try using www.getfirefox.com, and ensure you turn off any anti-virus or firewalls while you download.

Tried already. Can't solve the problem.

Try downloading the required version from http://releases.mozilla.org/pub/mozilla.org/firefox/releases/latest/win32/

There are in fact methods that may be employed to check a downloaded file is not corrupted (The directory above the linked one provides various hashes) but maybe your error message will give us a clue to what is happening. Please say exactly what the error message is and when it appears, preferably attach a screenshot (use the Add images option under your post a reply box)

• How do I create a screenshot of my problem?

Attached is a pic of the error message.

You can find the latest Firefox release in all languages and for all Operating Systems here:

• Firefox 16.0.x: http://www.mozilla.org/en-US/firefox/all.html

Which security software (firewall, anti-virus) do you have?

It is possible that your anti-virus software is corrupting the downloaded files or otherwise interfering with downloading files.
Try to disable the real-time (live) scanning of files in your anti-virus software temporarily to see if that makes downloading work.

See "Disable virus scanning in Firefox preferences - Windows"

• http://kb.mozillazine.org/Unable_to_save_or_download_files

I am using avast antivirus. I tried disabling avast, windows defender and windows firewall but the problem is still there. All the downloads are corrupted.

Try to boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test to see if that helps.

• http://windows.microsoft.com/en-US/windows7/Advanced-startup-options-including-safe-mode

Has anyone checked the task manager to see what processes are running ? ctrl/alt/delete.

Ive the same issue and cant seem to get round it 
Ive tried stopping most processes what arent being used.

Im using windows 7