it is a virus my friend
yahoo virus
and u cant remove it only with format
try this ites my friend and scan online ur pc maybe u will solve the problem
*norton
http://security.symantec.com/sscv6/default.asp?langid=ie&venid=sym
*housecall
http://housecall.trendmicro.com/
*kaspersky
http://usa.kaspersky.com/products_services/free-virus-scanner.php
*mcafee
http://us.mcafee.com/root/mfs/default.asp?cid=9059
*bitdefender
http://www.bitdefender.com/scan8/ie.html

good luck my friend