thanks for your answer, ok first of all, i installed raspbian using Win32DiskImager because i didnt have access to my linux box, im going to try again using dd this time. No i didnt redownload the image because i thought that the SHA-1 checksum was enough to be safe
I didnt even know about this Raspbmc, i should give it a try, at least to understand if it's an hardware problem.
The power supply is brand new, it declares +5v and 2100 mA, i chose it because of the 2,1 A, much more than the required value. Anyway i will check the tensions to see if 5v are provided, unfortunately i need to wait until monday.. i will let you know