where.this.takes.us

Would anybody have an idea about why this is happening when I go to modules->install/uninstall

PHP Fatal error: main(): Failed opening required '' (include_path='.;c:\php4\pear') in D:\Inetpub\ ... \administrator\components\com_installer\admin.inst aller.php on line 38

and that's all that I see. This doesn't happen with 4.5.1a on the same server.
I took a bit out at the " ... " for security

Thanks in advance