Microsoft Office 2007 includes a tool called Microsoft Office Document Imaging which has OCR feature which will allow you to extract text from tiff and mdi files. This tool can be found under Microsoft Office Tools. If it is not there you have to install it using your Office 2007 media.
I think this feature was also provided with earlier version of Office XP and Office 2003, but many might have not come across. I decided to give it a try.
I just extracted text from a tiff file and saved as word document. When I compared , it was almost perfect with few mistakes which you also expect from other OCR software generally provided with your scanner.
A nice feature provided with Office 2007 which can come handy when in need.