It wont let me install windows live messenger even though it says it on my laptop to install it, it

It says unknown publisher, and Also says,

this file was blocked because it does not have a valid digital signature that verifies its publisher

=[ So i dont knbow what to do, HELLP PLEASE x

It wont let me install windows live messenger even though it says it on my laptop to install it, it says..?windows live

What is the program thats telling you it was blocked,, was it the firewall?? was it an adware tool?? please let us know what program is blocking it?? if it is your firewall then go to your firewall settings and tell it to allow it,, If it is windows defender then open that and tell it to allow it,, Is this vista?? or is this an XP? ill check back as soon as I can,, Here is one more thing to check also,, go to tools at the top of your i/e page and click that then select internet options and then at the top you will see security,, click that and then set it to default settings,,