@pietrogiordano
Hi there, welcome to the forums.
I have had this issue myself and I have edited the template to fix it. It looks like you are using or falling back to the rwd theme. you will need to copy the file from the base theme into your active theme.
copy
app/design/frontend/base/default/template/customer/form/login.phtml
to
app/design/frontend/YOURPACKAGE/YOURTHEME/template/customer/form/login.phtml
You can then move the button HTML in that template.
Hope that helps. Please feel free to leave feedback.
Thanks,
Gaz.