The standard Yahoo sign in page in the link below has a box which will "Keep me signed in" for two weeks.
If you checked that box, then your login will be remembered for two weeks or until you sign out or...until you delete the cookies associated with Yahoo from your Temporary Internet Files folder.
So in addition to cleaning out your cookies, make sure you've made the correct selection in the "Internet Options-Advanced tab" in the "Security" section to "Empty Temporary Internet Files when browser is closed".
As to your last statement, it's not very clear, "when I click sign out from the yahoo mail page to long in to my other account, it's signs me out from the previous one". Isn't doing exactly what it's supposed to do?, Isn't it supposed to log you out?
Hope this helps.
I have updated my IE7 to IE8,
when I sing to my yahoo mail, and then click a new browser it comes with the same mail page, but in IE7 it does not keep me signed in yahoo mail,
and when I click sign out from the yahoo mail page to long in to my other account, it's signs me out from the previous one,
how to slove this problem please?
kindly any information to fix this trouble I will highly appreciate it,
many Thanks in advance,