Have you tried downloading it off Steam? But before you do that do the following below.
1. Reboot your computer and try again.
2. Your Temporary Internet files might need to be cleared.
1. For Internet Explorer users: Click on Tools, select Internet Options and
click on Temporary Internet files (or "Delete...") from the General tab.
Select "Delete Files". This may take several minutes. Once it's done, download
Combat Arms again.
2. For Firefox users: Click on Tools, select "Clear Private Data" and then
click on "Clear Private Data Now". Once it's done, download Combat Arms again.
3. Your download manager could be corrupting the downloaded files. If you have
a software that you use to manage or accelerate your downloads, we suggest
that you turn it off and download Combat Arms without it.
4.Disable or turn off your virus/security/firewall programs. If you have Norton,
it has the option to disable for a certain amount of time.