The requested URL /wp-admin/admin-ajax.php was not found on this server.

I get this error message suddenly

The requested URL /wp-admin/admin-ajax.php was not found on this server.

now before I’m told to “check” if the file is in that location it is. It’s a fresh install of 2.3.3, not an upgrade and I’ve dumped the entire database, cleared everything, and reinstalled it. I tried the .htacess file edit for the other error code (knowing it wouldn’t work but hey, it’s worth a try) and nothing. I have three sites using WP and this is the first time I’ve come across it. Very perplexed.