Virus in Microsoft Security Essentials

I need to find out what to do about Microsoft Security Essentials.  For some reason I no longer have it on my computer.  I did, but when I went to use it it was gone.  When I tried to reinstall it I'm being told it has a virus and it won't install.  I don't have a virus program or anything, I was using this for my virus and malware detection.  I read where someone was told to try installing it in Internet Explorer,  I did, that is how I found out it had a virus.  IE won't install cause of virus.  I tried in Firefox and it just couldn't install it.  But like I said  when I tried downloading it comes back with virus.  My question is where can I find  a copy virus free?