Forgive me, but talkin about network... If a hacker has your IP, what can he do to your machine if you are not firewalled?

portscan, identify your OS and then... run various exploids on OS-specific or service specific vulnerabities.... then install a trojaner, backdoor or just log in?

Excuse my ignorance (we could entitle this "Carlo learns how internet functions") but I naively thought that I had to run a prog which accepted incoming connections and accepted to download files. If my machine does not respond, how can someone upload something not requested by my modem?

The funny thing is your machine does respond!

I think it would be hard to explain every detail... but basically your computer does communicate with others. If a service has a bug (and software has many bugs, the more complicated, the more bugs has it following experience or Murphy's Law) then you can missuse this part of the software. You would wonder how many services or vulnerabities a normal computer offers. If you interested learn more about internet and operating system, protocolls, services... think geek!

"Think geek"?

It means to be a computer freak, a nerd, a geek. When you finally think like a computer freak don't forget to visit http://www.thinkgeek.com/ - awesome stuff!