There are 3 possible answers to your question.
A) If your WiFi is indeed protected by WPA2 and no one has the password than you are perfectly safe with the assumption that you trust whom ever is on your network...now with that said there is one exception. That if someone skilled enough wants to enter your system then they will and its only a matter of time. However these people are rare...very rare!
B) Second, if your being protected by WPA...then the likelihood of your system being compromised increases with the same above information applying. However with one change, a bit less skill is required and some free tools with a skilled person behind them can compromise your network. Why do you think they crated WPA2...
C) If anyone is on your network at all with the password or they have a wired connection to your router/switch/hub you are completely vulnerable and have little to 0 defenses short of having a PC side VPN. Not to confuse this with a edge device VPN as that encryption does not take effect until the data hits the edge and is clear text as it passes through LAN.
Basically, once someone is IN your network and you don't have a VPN setup on YOUR MACHINE...they can see everything you transmit.
Now the question is un-answerable once we discuss Linux/MAC OS X/ Windows discussions of permissions and exploits ect...so no I wont spend 8 hours discussing exploits for those 3 platforms.