Steam always changes my screen resolution!

[Sammy]

Hi everyone,
so my problem is that Steam always changes my screen resolution from 1920 x 1080 to 800 x 600, if I want to play any Steam games. It happens on every game that I own and only in one game (Star Trek Online) can I change the screen resolution back to the original resolution and can play it after with no problems. Even if I want to start Steam it always changes the resolution and I need to change it back. A friend of me, who knows about the problem, said I should reset my entire PC to factory settings and if that didn´t work I should buy a new tower unit (it is only 2 years old). I don´t want to do that because it would be a big nuisance to extract every personal document out of it and later install everything back.

I would be very happy, if somebody knows any solution.

PS. English is not my first language.