The same thing happened to me yesterday and then again today while I was browsing the United Mileage Plus forum. And it happened even though I did not click any banner or ad; I only clicked on threads.
On both occasions my antivirus software popped up and said it had prevented some kind of download. It identified yesterday's virus as TROJ_DLOADER.XAA, and today's as TROJ_RENOS.FV.
I urge the IT folks at FT to look into this.