unable to find fusion.dll

Have not been able to load .net 1 service pack 1. Get the following error
message: "This application has failed to start because fusion.dll was not
found."

I don't have a PhD in computer science .... but seems that common sense
tells me that I am supposed to have a fusion.dll file somewhere on my
computer and it's not there. Where can I download fusion.dll, and where
do I park it within my file structure?

Thanks in advance.