Change your password right away and your secret questions. Now uninstall messenger and all ad-ons you have installed including plus). Have a look through your program files and see if theres any "weird" looking programs that you don't recognize. If there is any uninstall them.
Run spy ware cleaners and anti-virus programs on your machine and delete/destroy all bad files they pick up.
Restart your machine in safe mode and delete all the files from your program files that you didn't install.
Restart. Install windows live messenger from the official website then plus.
Restart.
Problem solved. (hopefully)
Edit:
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