In Snow Leopard if you try to install a program while another is installing you get an error message saying “Waiting for other installations to complete.” But what do you do if the message stays there after the program has finished installing?
Today I was stuck with this message even though there was no installation in progress. The computer won’t’ restart as it wants’ to wait for the update to finish first!
The first option around this is to ‘force quit’ the installer. Press Apple-option-esc then select ‘Software Update’ and ‘Force Quit’.
If that doesn’t work try deleting the following 2 files from your Library/LaunchDaemons.