[tonemoz]

Use regeditx trial and search your registry for each istap 3.60.x.xxx version and delete any keys that it find. For example start with 3.60.0.600 then 3.60.1.003 and so on. For every version that you had installed, there's normally two entries and both would need to be deleted. This is the work around I used to install manually with .exe files.
Using the msi installer may not require to clean the registry so you can give that a try first...

I searched for all entries with 3.60.0, 3.60.1, 3.60.2, 3.60.3 and also all entries with ista/p. I deleted all folders and values from the registry. I deleted all folders within program data. I was then able to reinstall ista/p 3.60 and updated to 3.60.3.

Thank you for you help, it is very much appreciated.