Fatal Error – can’t log in

This is a really old version of WP – 1.5. Know it should have been upgraded long ago. I didn’t because it’s installed directly on the server (rather than thru Fantastico), and it’s a very customized theme – was afraid I’d totally muck it up.

On to the problem – when I go to my log in page, http://2withspirit.com/wp-admin/index.php, I receive the following error:

Fatal error: Call to undefined function: wp_clearcookie() in /var/tmp/tmpX3FaIp/sess_dfae05bd6f8d0d6bcd3003048c69e6d1(441) : runtime-created function(1) : eval()’d code on line 445

The front end of the site is still visible and functioning.

Any help in figuring out what’s causing this or direction in how to fix it would be greatly appreciated.
Thank you,
Cat