Your success will largely depend on the configuration of their computers and your router.
Option 1) Windows file sharing. Hold down the windows key, press r. Type in \[ip address of their computer] and hit enter. The slashes must face this direction. 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).
Option 2) NET SEND. You have Windows 7, from which NET SEND functionality has been removed by Microsoft. 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. (utility: http://www.winsentmessenger.com/sent/). Enter a command prompt (win-r, cmd, enter.) and type “net send ipaddress message” (no quotes). People have had mixed success with net sending to ip’s, the consensus is generally that LAN ip’s work for net sending. However, it may not work.
Option 3) Upsidedownternet. There’s a well-circulated tutorial that requires a dedicated linux box for a router, so it’s not practical, but worth adding. http://www.ex-parrot.com/~pete/upside-down-ternet.html
Option 4) Firesheep. Install this program and wait until the target becomes active. Start monitoring in Firesheep and wait for them to access a supported site. Then you’re logged in as them and can do whatever trolling you wish. http://codebutler.com/firesheep (requires firefox on your computer).