Some Antivirus software such as Avast picks up Messenger Plus as a virus. However this notification is false and likely caused by the optional sponsor program that comes with Plus.
There is no virus in Messenger Plus and if it was, believe me it wouldn't get the support it does.
I suggest you disable your antivirus, install Messenger Plus (without the sponsor program, at
this screen
click "I refuse to give my support..." and follow the installer.
After installation, re-enable your antivirus software and you should experience no trouble.
Avast also has problems with this sort of thing a lot (picking up software to be viruses when in fact it is not). It may be in your best interest to change to another AntiVirus software such as Norton or McAfee with less flaws.