You need to be really careful about where you get your themes. There are a lot of free themes out there which have code for ads and malware. It is highly suggested that you use a theme from the WordPress Theme Repository, as those themes are regarded as safe. There is enough of a selection where you can find a theme that suits your needs.
If your host does a complete reinstall of WordPress for you (which they might do if your installation was badly infected with malware), you may lose the contents of your site. It's possible that they may keep backups of your site (especially if it's a well-established hosting company); if so, see if they can roll back to a date before you installed your latest theme.