Help please. Problem with SEO mod

I have just implemented the SEO mod. I have pushed up the files for version 3.51 (my current version of ProductCart) The hosting company had to make the changes to IIS 404 error handler for me as I do not have rights to do that myself. Since they have made the change, my site still loads but if you select any product from the nav bar, it states "The system cannot find the specified path". The site has essentially been disabled for ecommerce. The URL's have been modified, however, to a more search engine friendly wording so at least that part worked. Any advise would be greatly appreciated.

Thanks, Greg. I have contacted the hosting company and they have confirmed that the url handler was set to a URL and confirmed the URL was correct. They have now taken it upon themselves to return the site to its original settings for 404 error handler so it is showing a standard 404 error code page not found. My site is still not accessing any product links and I don't know what to try next. Any other suggestions?

Just wanted to give you an update regarding this problem.  My hosting site would not implement/try the suggestion above regarding the URL. Their solution was to copy the 404.asp file to the root directory  (that's the way they handle the 404 error in their company). So....... I gave in and properly moved/saved the 404.asp file to the root, changed the links in the code and all is now well after almost 24 hours with the site being down. I appreciate all of the help that you gave me.