You can install your Poser 8 content library in an external runtime. When you install Poser 8 there are four options for locating your runtime folder. I selected the last option, and installed it to a partition that's dedicated for content (so all my Poser 8 content goes to D:/Poser 8 Content).

Then I have lots of others, like D:/Hair, D:/Props, D:/Mil 4 Females, and so on.  

I think there are some Python Scripts that you might HAVE to install to the Poser installation folder on your C drive, but for regular content type stuff you can use an external folder.