The lock handle on my aluminium windows keeps falling off. The handle had previously fallen off when I first moved in a year ago and the landlord remounted it. Since then it has become loose a few times, but has been able to be screwed back in. Now the two screws no longer hold. The internal thread (?) on the window frame is only a few millimetres thick.
How can I securely remount this handle?
I am not sure of the screw size/type, but below is a photo. 
The screw you're showing is a self-tapping metal screw. It's designed to cut it's own channel into metal and grab hold by cutting it's own threads - but it's not meant for repeated uses because repeat uses can damage it's own threads, making its hole too big for it.
Take it to the local hardware store and get one that is slightly wider in diameter, and use it in place of the old one.
Another solution: fit rivnuts in the openings and use machine screws. Should be stronger than the original, once you have the rivnuts in place. Setting rivnuts is a bit of a pain without the right tool, so you might actually want to buy a kit even if you don't have immediate use for most of it.
This does presume there is space for the rivnut to expand and lock in behind the old screw holes.
You need to weld a piece of metal with thread on the existing hole location of the window frame. Than your problem solve.
It is a problem with older windows. I had them at my previous house. The newer ones have more meat, as they are designed to a better standard.
Suggestion - Move the handle along by 15-20mm. And use Loctite when you screw it in.
I have used pop rivets in the past to secure handles onto old windows. You would need to find the most appropriate rivet size to fit the handle.
Once you have done that, removing the handle will require drilling out the rivets.
