Irfanview will do this. The batch option to preserve folder structure is a bit tricky. Irfanview has a
forum where it is discussed quite a bit. There is also FastStone Image Viewer, which is easier, but not free for commercial use.
You could use Irfanview to convert Tiff to Tiff and select ZIP, LZW(which are both lossless), or JPEG compression (if you can accept a possible loss in quality). Either option should reduce each file's size by a large percentage. In the case of black and white scans with a lot of blank area, the files should be a fraction of their current size.
Of course, as Sandeep says, always test first...a backup probably won't hurt either...
