Choose Edit Sets from the Sets menu, select a set in the resulting dialog and click Modify. In the sub-dialog that appears, you can specify a startup key and check the checkbox to make it effective. Option , if held down as the Finder launches, closes any previously open Finder windows. Control can cause the Location Manager to prompt you to select a location. Although Control is the default, you can redefine it in the Location Manager's Preferences dialog, and since Control held down at startup also activates Apple's MacsBug debugger, you may wish to pick a different key combination.
Command turns virtual memory off until the next restart.
Command-Option rebuilds the desktop files on disks when they're mounted. This can happen when you insert removable media, or at the end of the startup process as the Finder launches. Holding down these keys while all your extensions load may disable some of them - it's best to press the keys between when you see your last extension icon appear and before the Finder launches. The mouse button causes the Mac to eject floppy disks and most other forms of removable media, though not CD-ROMs. The C key forces the Mac to start up from a bootable CD-ROM, if one is present, which is useful if something goes wrong with your startup hard disk.