when I try to access the ip i get a login prompt asking for username and password for "level_15_access" at 192.x.x.x
What box has IP address 192.x.x.x? You say the error is not from Apache, so IP 192.x.x.x is not the Slackware box. Could the error be coming from your router? If the error is only seen on the Windows client boxes, then you might have a stale DNS cache; try IPCONFIG /FLUSHDNS on the Windows clients.