Apparently, the QlikView installer contains both the 32-bit and the 64-bit version of the plugin/ocx client.
Have a look on your server in C:\Program Files\QlikView\Server\QlikViewClients\QlikViewPlugin. The executable QvPluginSetup.exe installs on anything up to Windows Server 2008 and Windows 7 (including XP).
I just managed to install a plugin from a 11.20SR6x64 server on an 32-bit XP machine. No complaints.