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Another thing to do is to load the page and then review the returned code and look for the offending code which might hellp a little bit. Also, re-upload all your theme files from your backup.
The last one I saw used a bunch of ASCII coding for it, but I've seen standard HTML code inserts as well. Awhile back there were some exploits found, and among other things, I found that one of my themes had been hacked as well as the index page.
Updating XOOPS and installing Protector modules are absolute necessities. And look in your theme files and such (likely in the main page of the theme). It's unlikely to be in the database, but don't quote me. And, yes, empty the cache and templates folders (except for index.html).
Still broken? Post again with any additional info.