What are you doing when the XaReg error occurs ?

I would need the complete text of the details of the error to determine what has happened. The error indicates something is happening with the treeview which is used to list the installed versions of Xtreme and Web Designer. The out of range error could indicate that the registry contains entries for the programs but there is an error in attempting to retrieve some information. This could be due to a corrupt registry.

If you were using the recent file list option it is likely that there is a registry entry for a file but the file does not exist in the location the registry has listed for the file.

BTW there should never be a case where you would need to uninstall and reinstall any version of .NET. The technology is self repairing in the event of corruption.