If you are on Vista, go to ComputerProgramFiles. The user file should be there, I think; I have two partitions and the user file is in the Program File folder. I  had this same issue and had to uninstall/reinstall after deleting EVERY Poser 7 folder there was on my machine...turns out there were 3! If deleting the pref files don't work (as it didn't for me), uninstall/reinstall may be the only way to get it working again.

Might be a good idea to delete registry files too, but ya need to be careful with that.