ALL files on your HDD will still be available:
1. As far as "random" files go (photos, mp3s, video, .doc etc.), they will of course still be there and they will be fully usable as soon as you install appropriate programs to read them- MS Word (or OpenOffice), your favourite video player and so on.
2. Programs you had installed on the HDD will NOT be usable. For example, if you installed a game on your HDD before you reinstalled Windows, the game's folder will still be there, but it will be useless and can be deleted. You will have to reinstall any programs/games you need so that Windows can create the additional folders in My Documents, add file extensions and keys to the registry etc.