A new Heuristic virus which dosn't being detected in past but the file already in my pc for few years

Today i ran a full system scan and found that there is a heuristic virus in my hardrive (internal), however, the file (rar file) that contain that virus is installed in my pc since 2 years ago and i ran many full system scan before today and it seem no problem at all. But today suddenly being defined as a heuristic virus. I really afraid since my pc getting hacked a month ago and I already format my OS, I wonder that whether the malware of hackers infected other file in hard drive or just any other possible situation. Also, today I ran a custom scan of my external hard disk and found another virus which is Heu ADvML in my file (exe file). However, I have an excatly same file on the hard drive which scanned to be no threats. Why only the file in my external hard disk being defined as virus?  Can someone answer me? I really afraid since my pc being hacked in Aug.