Microsoft tell us how to remove the accounts option from Outlook Express:
To remove the Accounts option from the Tools menu:
1. Start Registry Editor (click Start, click Run, type regedit, and then click OK).
2. Locate the following registry key:
3. On the right-hand side of page, double-click the No modify accts value.NOTE: If this value does not exist, on the Edit menu, click DWORD Value. Type No modify accts, and then press ENTER to save the name.
4. Change the setting of this value from 0 to 1, and then click OK.
5. Quit Registry Editor.
So it's reasonable that the reverse will work to restore the option.