500 Error on running install.php

Hi,
I have just installed the new version on my hosted server, or should I say not installed it! For some reason it doesn’t seem to be connecting to the database server……….. I’ve double checked the config file, and I did this correctly once before so I am double-checking against the old one, which still works! Can someone give me a list of things to try – I’ve gone through the installation forum to see if the threads have any helpful ideas, but aside from one post saying make sure I connect….