Thanks for ur valuable suggestions & i am sorry i was on a vacation so i could’nt keep the track of the issue here. As Doris said i ran a HiJack scan on my system & am pasting the log here. & dear Dermot i suspect it to be a notorious virus than any IP conflict issues b’coz multiple log-ins on a single pc can be done with yahoo messenger using a plugin called yahoo multimessenger & i used to simultaneously use yahoo MESSENGER on all the pcs in my LAN before. I updated my yahoo too but , the problem still exists. AFTER I ENTER THE ID & PASSWORD THE MOMENT I HIT ON THE LOGIN BUTON THE ENTIRE YAHOO MESSENGER WINDOW VANISHES, EVEN FROM THE SYSTEM TRAY.

I have a attached the scan log from HiJack on one of the pc here

And lemme thank all u ppl again for keeping intrest in my problem.