I have WIndows 7 Ultimate and have this problem
If I download a .zip file I sem to allways get error saying file currupt, If I download and .exe file sutch as the new iTunes from Apple website I get to the Open File Security Warning then the install just stops (UAC allready on the lowest setting) BUT if I download from a torrent site things work fine!
If I try a direct install from the browser either Chrome, Firefox or IE it tells me the certificates are not supported/reccognised and just stops the install with no other message
I have tried loats of things including
UAC to lowest setting
Install as Administrator
Trouble shoot compatibility etc