Mystery file owner

Files on my computer are showing an owner identified as S-1-5-21-1427208717-013397328-3700319278-1001. I had seen this before on my removable hard drive where folders and files with gibberish names appeared on their own and the owner was identified S-1-5-21 followed by another long series of numbers. Initially I was unable to delete the file files. Eventually I figured out that I could remove take ownership of them, remove all the permissions from the mystery owner and then delete the files.
I have all my data files from my old computer on a removable hard drive (Seagate FreeAgent Go with 320 GB). I decided to keep my data files on removable disk after a hard disk crash. Subsequently my replacement disk crashed and I decided to replace the computer (HP Pavilion 1600) because I felt it was obsolete.

I now have a 2-week-old Dell Inspiron 660 running Windows 8 but I have used my removable hard drive and flash drive on it. I’ve heard that this S-1-5-11… issue is the product of a virus that is not easily detectable.

I am now concerned that that I may have infected the new computer.
Our network links 4 computers. An Xbox and a Roku player are also connected to it and we have 3 smartphones and other devices that are hooked up to the computers on a regular basis. I’m concerned about the vulnerability of our equipment. We have Avira antivirus, IOBit Advanced Care System Pro, and Super Anti Spyware on the two computers that run Windows XP. The two that run Windows 8 have only Windows Defender so far. Should they also have the same as the others?
Any help would be greatly appreciated.