Sorry to say this but it’s really your own fault. Not only are you using an unsupported version of Joomla, but an old version in that series as well. The latest version of the Joomla 1.5 series is 1.5.26 so there should be no reason why you weren’t using that.
I answered a question regarding being hacked and gave some advice along with recommended extensions to use. The guy was using Joomla 2.5 but it can still apply to Joomla 1.5.
Joomla! 2.5.4 Hacked: Having trouble with diagnosis
So remember, in future, keep up to date else it’s to your detriment.
Update:
To upgrade from Joomla 1.5 to 2.5, install this extension. This will update the core Joomla stuff and database tables. Do bare in mind that it will not upgrade 3rd party extensions, therefore you will need to upload the 2.5 compatible versions yourself.
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solved How to migrate joomla 1.5.18 to 1.5.20 – hacked [closed]