I use XP (SP3) and I just had a look and I don't have that registry key. Have you tried deleting the key altogether?
Guess you have done this in reverse:
microsoft KB click here it says it is for W98se?
"To remove the Accounts option from the Tools menu:
Start Registry Editor (click Start, click Run, type regedit, and then click OK).
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.
Change the setting of this value from 0 to 1, and then click OK.
Quit Registry Editor."