Believe me, if properly set up, closed all those ports which needs to be closed, you don't have to worry that much... anyway, do you have any information in your computer which is unbelievable important that no one outside should see ? I don't think so, after all you are not an secret service or anything like that
If your answer is yes, then I wouldn't keep those information in there if I were you... all those information, data, and other stuff you should keep on some DVDs, CDs, or even spare hard drive.
People worry too much sometimes, so I would say, we are never 100% safe, but behind a hardware router/firewall and software one as well, you are pretty safe and you can sleep tight...
Cheers !
EDIT: Also you can help yourself as well... do not surf those suspicious sites with patchs, cracks, serials, XXX sites, casino sites... be careful whe using P2P programs, enable avast! P2P provider for the P2P client you use, etc...