There is no 64bit version of the NSIS (Nullsoft Scriptable Install System). 64Bit processes can only be terminated from 64bit processes, so all the available NSIS plugins, such as the KillProc plugin, do not work. Additionally they do not kill the processes of all users – only those of the currently logged in users – which I wanted as well.

The problem is easily solved by packing an executable into the Installer which is then extracted and called. It is used like this in the UrBackup installer:

${If} ${RunningX64}
File "data_x64KillProc.exe"
nsExec::Exec '"$INSTDIRKillProc.exe" UrBackupClient.exe'
${Else}
File "dataKillProc.exe"
nsExec::Exec '"$INSTDIRKillProc.exe" UrBackupClient.exe'
${EndIf}

The source code of that executable is available here: http://download.urbackup.org/KillProc_src.zip
The binary files here: http://download.urbackup.org/KillProc_bin.zip
It expects only one parameter: The name of the running process to kill.