Okay, things like this I usually suggest taking to Tech support to resolve but it's late and you'd have to wait until tomorrow, which sucks - so here goes:

Install Poser 5.
Run the Service Release to update Poser 5
Start Poser 5 via the Start Menu or the desktop Icon.
Enter your serial number if asked to do so.
That's it.

If it still doesn't work, you'll have to contact  Tech Support, as there may be a larger issue involved.  The updater only works if Poser is already installed, so installing Poser after running the updater  (which is what it sounds like you did) will not resolve the issue.  I hope this helped.  Feel free to IM me if you need more help, okay?    :) 

Hope that helped.

-T