Yankee, you need to look at the location of the files when you get the report. There are several variants of the "fake antivirus" infection and there are several different trojans that hide and continue to download the malware. The installer is probably triggering an infected DLL on your computer that is only in use during an install process so don't turn off your antivirus software.
Make sure you have the latest version of your antivirus software in addition to the daily pattern database updates. This sounds like a really good time to run a full scan on your computer. Even though you already have Symantec antivirus on your computer, it never hurts to scan with a couple other online AV products when you know you have had an infection.
I browse the av-test.org (independent lab) latest testing reports and use the free online scanners of the top ranked software companies on their list. Occasionally the scanners will find something on one of our laptops that our antivirus software missed.
http://www.av-test.org/certifications.php