Trick for multiple Yahoo Messenger

First click start and run
type regdit
expand HKEY_CURRENT USER > SOFTWARE>YAHOO>PAGER> AND TEST
ON RIGHT SIDE
MAKE A NEW STRING IN TEST
NAME IT "PLURAL"
DOUBLE CLICK PLURAL
SET VALUE TO "0"
CLOSE REGEDIT.

NOW YOU HAVE ANY NO . OF INSTANCES YAHOO! MESSENGER WITH DIFFERENT ACCOUNTS SIMULTANEOUSLY