I have already installed IMVU, but when I go to start, it says I need to upgrade it, but the upgrade crashes the program. What do I need to do?
Try using Revo Uninstaller. Even if it can't uninstall the application normally, it will force it to uninstall by wiping out all traces of its presence from the system.
Once the uninstallation and cleaning part is over, download the updated client from the website and install it. It should work this way.