Windows cannot verify the digital signature on this file

Firstly apologies if this has been raised before but I've searched and couldn't find anything.

I'm running NIS 2010 & I've recently updated from Vista to Win 7.

 

Now, when I try to open/execute a downloaded program I get this message - "windows cannot verify the digital signature on this file. A recent hardware or software change may have installed a file........"

 

Has anyone seen this and is it somehow connected to Norton protection?

 

Any feedback & assistance would be appreciated.

 

Many Thanks

 

Ian