The Shift key should be held as soon as possible after the startup tone, but not before the tone.
The instructions provided by Apple are as follows: Boot in safe mode* and log in to the account with the problem. Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards. The purpose of this exercise is to determine whether the problem is caused by third-party system modifications that load automatically at startup or login. Don’t be disappointed when you find that nothing has changed after you complete it. Please read this whole message before doing anything.
