I am having the exact same problem. This just started with a recent update from Avast. It obviously now thinks the windows setup file is a trojan.
And why can't I override the stupid scanner? Yay, it thinks it found a virus, why can't I choose ignore instead of it telling me my only choice is to delete or repair?
Well, the antivirus can take some false positives from time to time. Not a software is perfect. But this does not make it stupid. It's doing it's job.
To know if a file is a false positive, please submit it to
VirusTotal and let us know the result. If it is indeed a false positive, send it in a password protected zip to virus@avast.com. VirusTotal has a file size limit of 10Mb. Please, mention in the body of the message why you think it is a false positive and the password used. Thanks.
Maybe you need to disable
Hide protected operating system files and enable
View hidden files and folders to manage the file(s).
As a workaround, you can add these files to the
Standard Shield provider (on-access scanning) exclusion list.
Left click the 'a' blue icon, click on the provider icon at left and then Customize. Go to Advanced tab and click on Add button...
You can use wildcards like * and ?. But be careful, you should 'exclude' that many files that let your system in danger.
This link is a tutorial on how to help correct a virus detection that you believe to be false:
http://forum.avast.com/index.php?topic=25009.msg204838#msg204838or
http://forum.avast.com/index.php?topic=7779.msg62586#msg62586