Ran into the same problem myself.
Fresh install, Windows 7 64bit
Admin prompt, UAC first on then off
Progress bar just kept going.... had to kill it, tried several variations of command prompt, elevated command prompt, local admin vs domain admin...
tried running lastpass_x64.exe -d got error can't create c:\lpdebug.txt required privilege not held.
Finally gave up and went back to https://lastpass.com/misc_download.php
and went down to the Old Releases link and
downloaded and successfully installed https://lastpass.com/releases/lastpass_x641.80.0.exe