How to update Paint.NET when asked for a missing MSI in the Staging folder:
Find out the version that is installed. - Mine is version 3.5.5
Go to File Hippo and download your current version.
You need to unpack the MSI from the downloaded EXE... run from a cmd.exe window (or Start Run): Paint.NET.3.5.5.Install.exe /createMsi CHECKFORBETAS=0 DESKTOPSHORTCUT=0
Open the folder created on your desktop, PaintDotNetMsi, and rename the filePaintDotNet.x86.msi to PaintDotNet_753582080.msi or whatever filename Windows Installer is asking for.
Copy the MSI to C:\Program Files\Paint.NET\Staging
Re-run the Paint.NET update - or uninstaller.
Whilst this issue is occuring it might be a good idea to supply the MSIs for previous x86 installations so that people with the problem can download them, rename them and add them to the Staging folder... (cutting out some of these above steps)
Oliver