I have encountered this myself, it turned out in my
case that the other person logged on was my son who was
using his video game which shares my internet cable.
If there are no such other devices in your case, you
might wan't to check your virus software and your antihack
utility. If you do not run personal firewalls (you
should) you can go to symantec.com and run free computer
security and virus scans.
