Inline tasks come to mind. Here's your example with inline tasks:

,----
| * Installation of PC X
| 
| ** Hardware configuration
| blah,blah,blah,
| 
| ** Software configuration
| I intall app x, no trouble
| 
| I try to install app y, but no luck
| *************** TODO App Y is giving trouble, fix
| Details about trouble...
| *************** END
| 
| I install app z, no trouble
`----

The command to insert an inline task is 'org-inlinetask-insert-task'; I
believe that there is no default binding.

Regards,
Georgiy

On 08 Oct, Angel de Vicente wrote:
> Hi,
> 
> I'm starting to use org for notes, and I realize that I don't know how
> to create a TODO item but without it being an outline item. Sorry if
> this is not the proper terminology. Let me explain what I would like to
> have. Let's say I have a file where I want to keep details about a new machine
> installation. I would have something like:
> 
> ,----
> | * Installation of PC X
> | 
> | ** Hardware configuration
> | blah,blah,blah,
> | 
> | ** Software configuration
> | I intall app x, no trouble
> |
> | I try to install app y, but no luck
> | *** TODO App Y is giving trouble, fix
> | 
> | I install app z, no trouble 
> `----
> 
> For the "software configuration" I just type notes of what I'm doing,
> and I would like to have TODO items for unfinished tasks.
> 
> If I write something like above when I collapse the TODO item, all the
> information about the app Z collapses with it. I know that I could
> create another outline level for each of the apps (or at least for "app
> y" and "app z"), but that will force me to create extra levels whenever
> I want to insert a TODO item in the middle of a section.
> 
> So basically I would like to have that TODO item but somehow without
> creating a new section. Is it possible?
> 
> Thanks,
> -- 
> Ángel de Vicente
> http://angel-de-vicente.blogspot.com/
> 
>