Vista - Offline Contacts Sending Links To Porn/Virus?

I have a problem of people in my contacts who I know ae not online but messenger is sending me message(s) to open from the as tho they are online hich link to porn site game sites etc & it is automatically sending to my other contact as tho its from me, my antivirus sweep has come up clean any ideas how to get rid of this problem/virus also my messenger says i have signed in at another location & I know i haven't.


This is an urgent request for help!!!!