.... I'ver changed my pc re-install the Trados program downloading again the licence file, but when I open the file it appers a windows in wich you have to choose if buy a licences, get her online or downolad from your pc. BEcause I've the licence file I select it and the program says that it's ok, but when then appears a window telling "you should have a valid licence to run SDL Trados 2009"
Hi I don't know what you mean by "license file" because there is no such thing as a license file for Studio. The activation happens online and if you transfer Studio from one PC to another, you need first to return the license on the "old" PC (you do this from within Studio under "Help - Product Activation" by clicking "Deactivate"). This frees your license so that you can use it again to activate the installation on the "new" PC.
If you don't return your license, you will not be able to activate again because the license is still in use.
So, I suggest that you return your license on the old PC. If you can't do this anymore, because you uninstalled Studio or because the PC has already been wiped or you don't have no access to it anymore, then you should be able to return the license from within your SDL user account under "Licenses" using the self-service deactivation (this works only for versions up to Studio 2014, but if I understand you well, you still have Studio 2009).
If this doesn't work, then you need to ask SDL to reset your license. You can do this by going to support.xl8.one and filling out the form.