This happened to a client of mine recently that was hosted on ipower. I'm not sure if your hosting environment was Apache based, but if it was be sure to double check for .htaccess files that you did not create, particularly above the webroot and inside of image directories, as they tend to inject some nastiness there as well (they were redirecting people depending on where they came from in the refer). Also check any that you did create for code that you did not write.
