That could be the name of your homegroup you've setup for the windows machines, failing that someone has accessed your network and homegroup, and your seeing there computer, try jumping into the files and folders and see if you recognize any of the files in there.
Failing that, log into your router and check the active connections against the mac addresses of your machines, and ensure no one has gained access to your network, log off all clients that you don't approve off and change passwords.
If you have any more problems, please feel free to email me on my profile - I don't always check back for edited questions, so you may want to Email me for a quicker response, or just to alert me that you've edited the question. I'll be more then willing to help you through any process and remote assistance can be used if needed.
Edit. - Was not aware that Emailing on personal addresses was a violation of the rules, thanks for the heads up there! I'll ensure to have people mail me from my profile.