If someone poll your PCs, the message will appear. Don't panic, this DOES NOT mean that your PC was being used by someone else. If you did not SHARE any of your resource (drive, directories or printer) to the network and you have a updated virus scanner, it is safe.

With the new virus like CodeRed/Nimda, more and more users are polling the network to check if others is infected, you may see these message every now and then. If you disable the file/printing sharing option in your PC, you will not see the message but the drawback is that you cannot share your file to your peers.

Procedures to enable/disable File and Printer Sharing.