Just a suggestion: could the calibre installers (wherever possible) request the user quit calibre before continuing installation? I keep on falling for it.. :)

This catches me out too, especially because the close box (top right of the main window) now minimise calibre to the system tray, so it is still in fact running (I know you can switch off the use of the system tray). Perhaps an Exit menu option is needed, which really closes the app (maybe it is there and I have missed it). I also know you can close to the system tray and then right click on the system tray icon and choose quit to close the app. But this is not the same as being able to close directly from the main window.

Please help.
The graphical user interface of calibre is not starting on Windows (XP).
calibre-debug -c "from calibre.gui2.main import main; main()" as it had been described in the Manual gives:
link hasn't been detected!
link hasn't been detected!
what can i do?

@AprilHare: You mean the linux installers? They dont work if calibre is running? The windows installer IIRC tells you to quit a running instance right at the start.

@anatoliy: Try turning off any antivirus/malware protection you may have

@ShellShock: Press Ctrl+Q to quit