Ok, let's try it one more time, with a different mirror.
First, clear your cache, then the downloaded files list (in Firefox).
Make sure there is no MsgPlus-354.exe in your computer.
Close your browser. All instances.
Open it again, and try from this mirror:
http://mimsn.com.ar/mirrorland/MsgPlus-354.exe (it's my site, I just downloaded the file and uploaded it there)