The Start menu in Windows 9x has a nasty way of growing out of proportion, creating unwieldy cascading menus. In Windows 2000, the Personalised Menu feature collapses the cascading menus under the Start button to show only the items you're likely to need. Windows does this by tracking the applications you've recently used or installed, and showing just them - the others drop off the visible menu.
To access the entire contents of a menu, click the down-chevron symbol at the bottom of the list. The whole menu defaults to an alphabetical listing, with the top-level items standing proudly on buttons.
If you don't know where an item is hiding, you may want to turn off Personalised Menus and display the whole Start menu. Select Start-Settings-Taskbar & Start Menu, and under the General tab, deselect Use Personalised Menus.