Thank you for replying, your suggestion gave me some food for thought, indeed!
I have already tried to call to my ISP and asked them about the problem. The answer, surprisingly enough, was: "We have no responsibility for p2p networks". What do you think?
And after installing Windows XP today and letting my emule run for 8 hours, I noticed the same thing, no upload this time at all and less than 2Mb of downloads, with sources a plenty. I turned the firewall off, and not a thing. I will try to use the Windows version of aMule now, and change the ports numbers.
Thank you again.
PS: When I try to download a file by http or ftp, the net speed is ok.