Update:
After a reboot to get rid of the hung installer, I tried again but soon after starting up the installer complains "Unable to find valid target platform" and ends.
Next by "ISSI instructions" I assumed you meant run the "Lotus Notes Client Removal Tool V2.0", which I did and it executed successfully.
However I am still getting the error "Unable to find valid target platform". I have even removed (well, moved) my c:\notes folder and I'm still getting the error.
I also ran the nice.exe tool. At first it would not do anything as it could not find a notes installation, but I ran it with the -wipe option, to clear the registry. Didn't help though.
Any ideas please? Do you want me to gather MSI logs?
Cheers,
Colin