The way to find the product code (not product key) is to download and run a small MS tool called msiinv.exe. Get the new version. It will list amongst others all product codes of "advertised" and "installed" programs. The product code for PND 4.05 was in my case {141BA46D-2D1F-4DA6-9448-B847334585C0}, (I am not sure if it's the same for other machines). However, there was the note that PND is "advertised but not installed, code 1". That is the reason why it is not in my list of programs and features. However it runs normally.
Entering this product code into FixIt, did not really solve the uninstall problem. Although FixIt reported problem solved, it turned out that PND 4.05 was still not in de Programs and Features list (not to be expected because "uninstalled" by FixIt), but could still run. Updating to 4.06 still gave the strange error of a missing msi, as before. I am at a loss.
Regards, Teunis
Added: Note that msiinv.exe runs in a cmd environment. It takes a long time to run. When it's finished it reports where to find the logs with the info.
1hour later:
Problem solved! I could now install 4.06 over 4.05 - without a hassle. Everything runs fine.