This site has just been defaced by a group named 'Spygrup'. Luckily i realized it very soon and was able to restore every change made.
Those dudes went in by replacing a special file in my Joomla distribution and thus covering the whole site.
They didn't damage anything else and no data got lost.
I reported this incident to my web space provider and hope they can find the security hole by checking the logs.
Well, here's a screenshot of the website as it was defaced:
By the way, the final version of the xp-AntiSpy compatible with Windows Vista is comming very soon.