go to c:\documents and settings\[username]\application data\mozilla\firefox and delete the profiles folder. Then go to c:\documents and settings\[username]\local settings\mozilla\firefox and delete the profiles folder in there. This will remove all your saved passwords and bookmarks, so copy down any username/passwords that you can't remember (go tools -> options -> security -> saved passwords) and make sure your bookmarks are backed up.
Unfortunately this didn't work either. It worked for one download only and anything subsequent gives the same problems. Must be something with Windows I'm assuming. Will have to switch back to Opera I guess Thanks anyways everyone