it requires changes in how Windows agent reads list of installed software. I'll try to fix that before 2.0 release.

Best regards,
Victor

I still had some issues with problem no. 1, as it "moved" from the DCI script to template auto-bind one.
However adding another line of code resolved that. I also changed the script it to only apply to Windows systems.
So now it looks like this and does not throw any more errors:

if ( (! $node->isAgent) && (! $node->platformName ~= "windows.*") )
	return false;

table = AgentReadTable($node, "System.InstalledProducts");

if ( table == null )
	return false;

colName = table->getColumnIndex("NAME");

for (currentRow = 0; currentRow < table->rowCount; currentRow++)
{
	if ( table->get(currentRow, colName) == "Microsoft Exchange Server" )
		return true;
}

return false;

Still not quite sure what to do in regards to installed Windows Features. 

So use this as an auto-bind script on a template, and you dont need DCIs or custom attributes.
It will also solve your issue no. 1.

if (! $node->isAgent)
  return false;

table = AgentReadTable($node, "System.InstalledProducts");

colName = table->getColumnIndex("NAME");

for (currentRow = 0; currentRow < table->rowCount; currentRow++)
{
  value = table->get(currentRow, colName);

  if (value == "Microsoft Exchange Server")
    return true;
}

return false;

To answer my own question somewhat, I managed to do what I had planned by creating a script that checks System.InstalledProducts, returns "OK" (no matter what) and assigning this to a DCI.
Along the way of checking that table, it sets custom attributes for any installed product that I want to monitor.
That then allows me to apply templates to systems with those custom attributes set.

Script below (for testing I only checked for Microsoft Exchange Server, but can be adjusted to other things):

sub main()
{
	table=AgentReadTable($node,"System.InstalledProducts");
	idxName = table->getColumnIndex("NAME");
	for (row = 0; row < table->rowCount; row++)
	{
 		value = table->get(row, idxName);
 		switch (value)
		{
 			case "Microsoft Exchange Server": SetCustomAttribute($node,"Microsoft Exchange Server","Installed");
		}
	}
	return "OK";
}

Now I have a couple more problems:

1. My NetXMS server now keeps throwing up alarms against itself all the time: "Script (System::SetInstalledPackageAttributes) execution error: Error 14 in line 4: Function or operation argument is not an object". "System::SetInstalledPackageAttributes" being the name of the script. The script is not actually called against the server itself, so this appears to be an issue when the server calls it against a node? This DCI is only applied to systems with the NetXMS agent installed and running Windows. All of which should have System.InstalledPackages?

2. This only works for software showing in that table, which doesn't include Remote Desktop Session Hosts or any other "Windows Feature". I could check for thos via (deprecated) WMI Query (which may also run into timeouts) or I would have to call PowerShell's Get-WindowsFeature cmdlet. Unfortunately I have currently no idea how to do that one in NetXMS... maybe an idea for a subagent? Together with uploading PowerShell scripts, that could be quite powerful?