Update for the Windows Installer

I have taken the time to try and make the windows installer a bit more sold, as it didn't work well with my windows system (where I run as a low-privileged user, and manually approve any escalation of privileges).

The main changes involve changing from the 'local user' settings to the 'local system,' and requiring admin privileges to install.  (of course any user will be able to run Bitcoin without being an admin still).

Of-course this will need more testing.