Hello!!
thaks for the reply!!!!
Eventually I managed to solve the problem in the "brute" way..
I marked my 7 partition as "Active" (which obviously wasn't) thus making the boot impossible ( turning on the pc it said "bootmngr.exe not found").
Then i booted from the 7 installation dvd, went to the repair section/start up problems, choose 7 frome the 2 operating systems found and let the program do its work.
When i restarted my 7 partition was the "system" one, and my vista partition a "normal" one.