Everytime I try to download Unity to my windows 7 computer it displays the message: Installer integrity has failed. Common causes include incomplete download and damaged media. Contact the installer’s author to obtain a new copy. I have also tried downloading older versions of unity but the same message is displayed every time. Can someone please explain what/why this is happening and how i can fix it? Much appreciated 
The 4.5 installer I downloaded is 1175723520 bytes, and has an md5 hash of
ac66d74f7c6eb1190bdde1d71b01f158. If you get different values, then unfortunately your download is corrupt. Maybe try to clean your browser cache and try again, or use a different browser.