I read that someone removed the file "firefox.exe.moz-upgrade" file.

SO I did this...but it still came up with the "reboot window"....

SO I went back into the downloaded file, opened/extracted it and went into "custom" setup.

Changed the destination folder to "/mozilla2" (or change to whatever so that its a different folder to the default).

Worked a treat after this.

I'll give it a try. I really dislike having to use IE.

Do a clean (re-)install:

• Download a fresh Firefox copy and save the file to the desktop.
• Firefox 4.0: http://www.mozilla.com/firefox/all.html

• Uninstall your current Firefox version and remove the Firefox program folder before installing that copy of the Firefox installer.
• Don't remove personal data if you uninstall the current version.
• It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.

Your bookmarks and other profile data are stored elsewhere (not in the Firefox program folder) and won't be affected by a reinstall, but make sure that you do not select to remove personal data if you uninstall Firefox.

Success! Turns out the program folder was hidden so I found it and deleted it. Did a clean reinstall and it works (fingers crossed!).

Thanks very much!