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Blimey - that looks well dodgy. Not a surprise to see it's a viagra company though!
I am fairly sure that to adjust the registration e-mail, they need access to a file in your webspace, namely
/language/english/mail_template/register.tpl
(
See the FAQ)
So to change that they must have hacked your actually web space as opposed to Xoops? No expert on that side of things though.