The "Insert Symbol" option is only there when you are
using Word as your email editor.
Is it possible that you've turned off that option without
realizing it? (In Outlook 2000, check in Tools > Options
Mail Format tab, see if "Use MS Word to edit email" is
checked off)
If you are in fact still using Word, try using Tools >
Customize in that program to return the menu to its
previous state.