Try updating via Help | Check For Updates

I have tried downloading Mozilla now from several sites, all of them giving the same problem. As I have deleted Firefox in the hope that this would clear the way to a successful upgrade, I cannot use Help.

Thanks anyway for the suggestion G

If you haven't emptied the Recycle Bin, the file you deleted should still be in there. Right click it and then click Restore. Windows will replace it in the same folder you deleted it from.

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

Thanks for these replies. Xircal, Firefox is not in the recycle bin (and no , I haven't emptied since having this problem). I have tried the download with Norton switched off, but no better

OK, do this please. Launch IE8 and go to http://www.mozilla.com/en-US/firefox/fx/?ref=logo

Click the Firefox 4.0 download button, or if you want it in a different language, click the link to "Other Systems and Languages" immediately below the green button to get the list.

When you get the option to install in IE8, click the Save button instead and download it to the desktop.

Go to Start | Run again and type appwiz.cpl and then click OK. This will take you to Add/Remove Programs.

Scroll down to Mozilla Firefox and click Remove. If it's not there, then it's not installed, but better to check first.

Next, disable your AV, usually by right clicking the systray icon for it and clicking Exit.

Then run the Firefox installation file you downloaded.

Should work.

Many thanks Xircal, for your help (and for your persistence!). You have helped me solve the problem.

You're welcome ;)

Oh, and could you click the "Helpful" button next to the post which fixed the problem for you?

That will mark it as "Solved" and saves someone else reviewing it.

Thanks.