I had this issue yesterday on one of my windows server. The problem actually is with Java.
Download the below java version and re-install the Download Manager.
http://filehippo.com/download_jre_64/tech/7930/
Also, don't forget to change the compatibility mode while installing Download Manager.
Right click on the Install.exe(download manager) icon and select properties. Goto compatibility tab and check the box "Run this program in compatibility mode" option; there select "Windows 7" and click Apply.
regards,
pavan