You might be experiencing an error due to a bad cache file, here are instructions on clearing out this file:
common_installation_issues.pdf
If you still experience the problem after doing this then there might be a corruption of your configuration folder. If this is the case you will need to follow the steps here to overcome this:
showpost.php?p=12293&postcount=2
If you continue to experience this problem after trying these things please let us know.