If you had the earlier version of MS PWS installed and then install the later PWS 4.0, it is possible for two copies of the asp.dll file to be on your system, and the older one may be loaded, causing some errors when you try and run StoreFront 2000 or other newer ASP files.
The solution is to search your hard drive for all files named asp.dll and delete the old version. Of course, make a backup, just in case.