I don't have a particular package to recommend, but OCR software recognizes text in images and converts the image to text that you can edit. I'm sure there's plenty out there you can try. I'm guessing some packages will try to handle handwritten characters as well as type-written letters, but i don't know the state of that sort of thing.
Yep, Lee is correct. Your only option is finding some software with OCR capabilities so it can "read" the document and guess what the characters are meant to be. OCR is always a bit hit-and-miss depending on the image quality and size of the file, so there's usually still some hand-editing that needs to be done. You can try doing a web search for OCR to see if there's any freeware out there, or if you're lucky enough to have Adobe Acrobat installed, you can use it's built-in OCR feature.
Just remember to always ask yourself this... Could you have just re-typeset the document in the time it took you to hunt down a "quick" fix? lol... I bust myself with this all the time...
A tiff is an image file. You'll probably be able to recreate the file faster in a program like indesign/photoshop faster than you could decode the tiff and edit the text. I would import the tiff into photoshop, edit out the text, then add back your own text in photoshop/indesign.
Depending on how long it is, if you don't have access to OCR software (acrobat, etc.), you should probably just retype it. You could try out Google's OCR, but I've heard it's not too great...yet.