To the original poster, I recommend Avast Antivirus. It is one of the most top anti-virus applications out there, and has won many awards for its detection abilities. With the new version and the overhauled interface I would recommend this over AVG. If your worried about Avast slowing down FSX you can modify the File System Real Time shield to exclude the Microsoft Flight Simulator X directory. If your want performance but want to be safe incase, you can modify Avast to only scan on file Reads, Writes, and/or Executes.You can get avast here:http://www.avast.com/indexReview of it by cnet:http://download.cnet.com/Avast-Free-Antivi...4-10019223.htmlWiki Page:http://en.wikipedia.org/wiki/Avast!Awards (Avast won the VB(virus Bulletin)100 award :http://www.avast.com/awards-certifications

ESET NOD32, as recommended by Nick Needham in his FS Tuning Guide. I use it myself and can highly recommend it, it's the best AV program I've used.

I also use ESET NOD32 and have since 2007, never had a problem!