Wild thing is, I need this file to be able to properly uninstall the HP printer drivers on this system but the file does not exist in any form on the installation CD, nor is it in the Win98 cabs.
A couple things.
First, If you are simply trying to uninstall, create a text file with jibberish in it and then renamed it to userenv.dll. It may be just looking for a file by that name and not care about date, size, etc.
Second, the unistaller may be determining what it needs to uninstall based on a .dat (or similar file). Trying identifying such a file in the applications root directory and removing that line from the file.
Third, it may be getting that information from the registry. Search the registry for userenv.dll and report what you find.