[Sweeks]

It sounds like your servers been hacked, in that case I would contact your host and make sure you change ALL of your password to your sites and FTP, Databases etc and scan your computer too make sure it is clean too. Make sure your files and folder permissions are set correctly and look in htaccess for any malicious code too.

Actually it might not be a case of your site being hacked, it could be a member linking to viruses and such within their posts or signature, in that case I would check through all active members signatures thoroughly. Best of luck