go here and test your online security
symantec security check then check and see what is exposed to the internet .
All your ports should be closed or in stealth (the only open port that is still safe to have open is the "ping" port .
pay close attention to the netbios
one more note : this test will always Fail your Antivirous program (even when running there own AV program it fails)
but with the security check ok, a good pop up stopper ,antivirous program , and a spyware remover (such as lavasofts adaware) you can keep your pc safe . (a firewall just tightens it down more but uses more resources and slows data up . unless its a good hardware type)