Permission Drupal folders
Most directories and files in a stock Drupal environment just need read/execute for the IIS user - this allows IIS/PHP to read and execute the .php, .inc, and other files. For your "files" directory - typically in sites/default/files - IIS will also need write permissions, so it can write uploaded files and create new files (for instance, ImageCache derivatives).

As for how to do this in Plesk, it will vary, but here's a guide that should get you pointed in the right direction (depending on your host, some of the user/group names may differ, but should be close):

http://knowledge.3essentials.com/web-hosting/article/478/Setting-file-folder-permissions-from-Plesk-control-panel-for-Windows.html