Here is the way but do at ur own risk. U must be Admin of ur machine.

Goto Run type regedit
Hit enter
Goto HKEY_CURRENT_USER\Software\yahoo\pager\Test
On right pane Right click and choose new Dwordvalue
Rename it as Plural
Double click it and assign Decimal value with 1
close registry
restart Yahoo messenger.

If it didnt worked try restart ur machine.
For signing in with new id open new messenger.