Explanation of this tool from Microsoft:
If you've installed FrontPage on a server, Open With FrontPage does what you would expect. Recalculate Web checks the hyperlinks on the Web site then rebuilds the site inventory based on what's in the Web site's directories, which is useful if you use a regular File, Copy operation to move content into a Web site (without using the FrontPage client). The FrontPage client won't see this content until you perform a Recalculate Web operation. (This same option is available in the FrontPage client under the Tools menu, but there it's called Recalculate Hyperlinks.) If you change any settings in the Microsoft Index Server machine or in the configuration of your search operations, Recalculate Web (or Recalculate Hyperlinks) makes sure that all the search-engine reference files update as well.
For more information please visit: http://www.microsoft.com/technet/prodtechnol/windows2000serv /technologies/iis/tips/administ.mspx