Click (or double click) on the splash screen to see if there is a hidden message underneath. If there is - forum search it and / or let us know what the message says. If there is none - boot from LiveCD and check the /home/user/.xsession-errors (of the installed system) for any obvious errors.
You can also try to log in as root on the installed system and check the /var/log/ files for any obvious errors. If the files are not "updated" make sure that the Syslog service is being started at boot time (PCC > System > Manage System Services) and reboot. Files should be updated with current system info once this is done.
Another thing would be to create a new user account (it can be done in PCC as well on the System tab) and try to log into the new user's account to make sure that this is not something system-wide but user specific.
For now that's all I can think of.