Shoutbox

If windows defender is blocking adware and you installed plus with the sponsor then this could potentially be causing the problem.

The sponsor is an option during install.

Try removing Plus! (from Programs and Features) and re-installing without the sponsor

Also, are you an administrator on your Vista install?  If not then it's advisable to run the Plus! installation as an administrator (right click on the file and choose "Run as administrator"....also try the same when opening messenger