Uknown User Account On My Computer?

Okay so if I right click a file> click properties> click security tab it lists the following Group or user names:
System
Lenny (LENNY8\owner) <<<< that one is me.
Account Unknown(S-1-5-21-875340621-3714830989-22…
HomeUsers (LENNY8\HomeUsers) <<<< I think that is the other computers on our network.
Administrators (LENNY8\Administrators)
WMPNetworkSvc
The unknown user has the following permissions:
read and execute
read
permissions.
The unknown user is only on some of my files. Like it was on one of my videos and on one of my documents but not on another document etc.
I am running Windows 7 Home Premium. 3 of our computers are also running windows 7 home premium and we are on the homegroup. Are 4th computer is running Windows XP Pro so it is on our network but not homegroup because windows xp does not have homegroup. I am the only account on this computer and Guest account is turned off.
I do not think I have a virus because my computer is running great. Although a few weeks ago I had a terrible trojan. I had to turn system restore off then run malware bytes and anti malware, then turn on system restore and create a new restore point. My security software Webroot Anti virus with Spy Sweep has not detected anything since then.
If you need more information please tell me so I can provide.
Thank You.
Additional Details
The unknown user is also listed on one of my documents about the security system but then the user is also listed on documents that you can tell are unimportant by the name such Library Card number. A couple weeks ago the credit card company detected charges on a credit card. Yesterday my mom noticed 2 $800 orders from staples on another credit card. I have been doing more online shopping in the past like on Amazon. Also Google Chrome is remembering all the credit card information.
4 days ago
Okay I was just on my sisters computer. She said a couple of days ago that she suspected she had a virus because her computer would freeze. I viewed the security tab of one of her documents. It froze and the unknown user was listed and then it started responding again and the unknown user was no longer there. If she did have a virus it has not been removed yet. Also today Windows Security detected a virus and removed it. Malwarebytes and Anti Malware has not detected anything. The unknown user accounts are still there even though windows security said it removed a virus. There are now 2 unknown accounts. Under the security tab where they are listed it freezes one acoount disappears and then it unfreezes.
Please help.