I am having a similar problem.  I have downloaded the client from both mirrors.  Both times the size of the file was 271,438,388 bytes.  Both times when I try and run the executable I get the message below.  I do not have any anti-virus software running.

NSIS Error

The installer you are trying to use is corrupted or incomplete.  This could be the result of a damaged disk, a failed download or a virus.

You may want to contact the author of this installer to obtain a new copy.

It may be possible to skip this check using the /NCRC command line switch (NOT RECOMMENDED).