When the "swen" virus infected someone's system, it made significant changes to the registry that caused it to be extremely difficult to be removed by hand. what were those changes and why did those changes make it so difficult to remove?

In certain cases, when the Swen executable is removed or renamed by an anti virus program without stopping to fix the registry, it then will become impossible to run any executable files on the machine. This happens because windows will be unable to find the file that is associated with any executables on it's hard drive.