I bet that right now someone is searching for these “hidusb.sys and mouhid.sys” over the internet. I also know that I spent some time finding them myself.
What are they for? If you want to install a new USB mouse or keyboard you’ll probably be asked to “insert” one of the two files to continue the installation.
Don’t try searching for them on your own PC, you won’t have them once XP is asking for them.