That's not an installer error, it's caused by McAfee, since it mistakenly identified the Couatl.exe as a virus, remove it from the temporary folder the installer creates, so the installer can't find it to copy in the FSX folder.
it seems that my Mcafee is thinking couatl is a virus or a bad file! I restore it but cant find a way to say its a safe file so it doesnt send it into quarantine!
That's a very bad design choice by McAfee that, in their standard Home version of the Antivirus, have removed ( previous versions had it ) the ability for the user to exclude a file from scanning. Only their Enterprise version has this feature, which is a shame, considering even the even free AVG has it.
I don't see any other option for you, than to install and use FSX with McAfee turned off entirely, or switch to a better product. The free AVG might be a good choice at this time.
And not, this is not a problem that affects only our products, see this post at Flight1's forum:http://www.simforums.com/forums/forum_posts.asp?TID=24061
For the same reasons, they also suggest not using an antivirus that doesn't allow you to exclude files from scanning, and suggest a different product.