For the last two years I've been using the built in Microsoft Security Essentials on my Windows 8.x systems without the first problem. Are you aware that just about anything you download from download.com is malware laden? You should be thankful you weren't able to install it. If you want a comparable antivirus program, AVG will work with Windows 8.
I finally got Windows 8.1 on my laptop. Wow, that was a lotta work! So I download Avast, and it says it can't run on this pc.
I went to the Avast site which directed me to download.cnet.com. Have a screenshot - big banner that says "This app can't run on your pc" when I double click the Avast_setup.exe file.. Deleted the file, downloaded it again and same thing.