Trojan.Adclicker :Clients PC showing infections from old file location

We have few PC's  popping up with Trojan.Adclicker virus. Some quarantined and some infected. Upon investigation of the locations, its found that these are all IE temporary cache or Firefox cache. But these files were found from their old files which were copied to the machine as part of backup when the OS was updated 3 years ago.

So why werent these definitons detected earlier? Why is it appearing now. These files have been residing in this PC location for past 3 years.

Some of these are updated to 12.1.RU6 MP5 and some are still 12.1.RU6 MP4.

Some cache flies show ClickFrauder. All are .js extensions and have the same SHA 256 values.

Why are these appearing now? What is triggering them? This has appeared since last 2 week