iTunes wont install using update or manual download! HELP!

having major problems with my iTunes. i currently have 16 videos trying to download and i keep getting error messages everytime. contacted iTunes support and they said i needed to update to the latest version of iTunes. When i then told them that it won't let me they suggested downloading onto a different system! hugely useful! the problem i am getting with updating iTunes is that if i use the Apple Update it comes up with an error saying "failed to install try downloading manually" so i do! download the installer files manually and then double click on it as usual. the installer check comes up and i click on run. then nothing happens! the run box dissapears but nothing installs and nothing else comes up in its place! highly annoying as you can imagine! anybody any ideas on how to fix this so i can finally watch my 16 shows!