You're off topic for this forum, which is dedicated to Windows Live OneCare. You may want to try the Microsoft Public newsgroups here: http://www.microsoft.com/communities/newsgroups/list/en-us/default.aspx
However, I suspect that you have either connected to someone else's wireless network or someone else is now using yours. Configure your router to be secure (per the router documentation) and confirm that you are connecting to your own network. If you still see the other PCs, contact your ISP.
-steve