By default in Windows 7, when you hover your mouse pointer over the "All Programs" menu or other defined menus in the Start Menu, after a brief moment Windows automatically opens the submenu. This assumes that whenever you hover your mouse pointer over a menu, you mean to expand it.
If you do not like this behavior it can be disabled.
1. Right-click an empty area of the taskbar and choose "Properties".
2. The "Taskbar and Start Menu Properties" multi-tabbed dialog box appears. Click the "Start Menu" tab.
3. Click the "Customize" button.
4. The "Customize Start Menu" dialog box appears. Scroll down and uncheck the "Open submenus when I pause on them with the mouse pointer" box.
Disable automatic submenu expansion in the Start Menu upon mouse hover in Windows 7
5. Click "OK" on the dialog boxes to close them.
Thanks to tink
for this tip.