This is information on removing the c99madshell hack that can get into some Joomla 1.5 sites. Most of this content is an original article of News Blog. I was only able to find one portion of this myself before finding their article, kudos and thanks go to them.
If you see the following Joomla error appearing on most Joomla pages including admin section
File Not Found The requested URL was not found on this server
If you have a list of spam links
then your installation has likely been compromised. There is a security bug in Joomla 1.5 allowing a hacker to reset your admin password.
You should take the following steps to get rid of the error message and secure your Joomla:
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