Windows Live Messager and Yahoo Messager Problem?

hi, i have a question about my Windows Live Messager and Yahoo Messager that is when i have sign-out of the messager, then after a couple of minutes to hours it keep automatic sign-in without me click the "Sign-in" button. i wonder why is that? is this because of a virus? or a spyware? but after i click "Exit" button, then it wont sign-in again, but if i just leave it like that, it will auto-sign-in again. please help me this. but i've scan with Kaspersky and norton symactec but didnt find any viruses or spywares. but i still suspicious there are viruses or spywares, cos according to my teacher and my own knowleagde only viruses or spywares can monitor my PC without me knowing.