Enter a command prompt (win-r, cmd, enter.) and type 'net send ipaddress message' (no quotes).
If your target PCs are running XP or earlier, you can download a NET SEND replacement utility and use it to send the targets a popup box with any message you choose. You have Windows 7, from which NET SEND functionality has been removed by Microsoft. If they have Windows file sharing enabled, you will now connect to their computer and see whatever files they're sharing (if any note that they may have password-protected these files).
Your success will largely depend on the configuration of their computers and your router.
