I too have run into this twice, with two different installs. The first was
the factor install of vista premium on a brand new machine, and then the
second was a reinstall because of this problem.
It happens whenever i try to install anything. I get "Error 1500: Another
installation is in progress. Please wait until that installation is finished
and try again." There is no other installation, and no instance of the
installer running.
This is becoming ridiculous. PLEASE HELP!!!
Thanks,
D