Apache 2.4 on Windows 2008 large file downloads are corrupt
I had this same issue, and came across this hoping for an answer.
I strongly suspect there is a bug in 2.4 as this didn't happen in 2.2
After far too much mucking around I finally got my case fixed.
I hope this works for you to I know it's a work around but it's better than nothing.
In my httpd.conf I enabled sendfile. So swap
#EnableSendfile On
with
EnableSendfile On