You can try deleting the airport preference files (located in Library/Preferences, anything starting with com.apple.airport), you could back them up first onto the Desktop if you'd lilke. Sounds like a software issue, though, so if that doesn't solve it, I'd recommend an OS reinstall.
The upgrade to 10.6.8, consequently, would replace those files, so that might solve the issue as well.
I've tried that but it didn't fixed it.
I've also tried to create a new user with no success. If I open sys pref, network, the "disable" button is not working, the airport is on but internet is offline. I've checked router's settings and dsl line and both areok. Any other ideas? Is a system restore the most obvious thing to do right now or it could be worth taking it to an apple store?
A clean system restore (by which I mean an erase-and-install using the original restore software, not an upgraded OS) is the best possible way to rule out any software issues from being the cause of the problem. If you don't have a reliable backup, or simply don't want to go that route, you could always use an external hard drive (or an 8GB+ flash drive, assuming you have either one of those laying around and available to be erased) to install the OS on, and then boot the computer from that. This way, you wouldn't be touching your computer's hard drive, and you still get to test it with a "known-good" OS. If you have a chance of doing that, I highly recommend it. If not, take it to the Apple Store, since that's the first thing they'll do at the Genius Bar (boot it from a KG-OS, I mean), and will be able to address the problem if it's caused by hardware.