Can anyone of the experts ID this small critter?
Vaguely yes. How big is it, and where did you find it (window, yes, but where is your window? SA, USA, KZN, KNP...?)
I bet that little fellow is about half a centimetre long. They usually are very small creatures, and I never have heard of one that is even 1.5 cm long. Feel welcome to surprise me by cutting the tail off a giant scorpion! (Only, as I like scorpions, make sure that the scorpion is dead first.)
That was a typical, unmistakable, example of a pseudoscorpion. They are members of the order Pseudoscorpionida, of the class Arachnida, and as such they have the same rank as the spiders, the Aranea. However the Pseudoscorpionida are far less varied than the spiders. They all look like tiny, tiny scorpions without tails. To invertebrologists they are an easily recognisable group, although not many people know much about many species of them because most of them are very small and all of them, when not wondering out other windowpanes, spend their lives in the dark and under bark, stones and so on. Sometimes in dusty rooms one may find them among books and boxes.
Apart from being harmless, they generally are useful, feeding on small pests like booklice and clothes moth larvae (small ones of course!)
There are many interesting things about them, including the fact that they produce silk like some mites and spiders, but the spinnerets are not in the end of the abdomen, but in the jaws. Generally a topsy-turvy lot. Also they do not use the silk for catching prey, but for egg cases, spermatophores and so on.
I hope that helps.