You may need to clear out your cache file with the instructions here:
common_installation_issues.pdf
If you still have this problem after clearing the cache file it is possible that there is a problem with the configuration folder. If that is the case you will need to follow the directions here to have a new configuration folder created for you.
showpost.php?p=12293&postcount=2
If the problem persists please let us know so we can open a ticket for you on this issue.