I haven't installed the most recent service release for Poser (came to the forums to see if there are any reasons not to), and my Poser library is working fine. But, if you haven't used Poser in months, then apple probably did impose a stop for the older versions. Adobe flash is one two primary sources for malware in OSX, and apple restricts which version of both of those two applications from running when they find vulnerabilities. I'd say it happens 3-4 times a year...or it used to. I think it's gotten better over the last few months. 

As for yahoo, I have never seen it myself, but I've heard that some of the adobe free installers (like flash or PDF reader) come with "free" software that you "might be interested in". This is stuff that other companies pay adobe to have added to the installer, such as yahoo's search bar. If you can't uncheck any of those, then you probably need to find another installer.