Penumbra Overture, frequent switch to my desktop

[flcn27]

I don't have any other programs running besides Steam.
I turned off the overlay and now it is working fine (so far). So I guess it's a problem with the Steam overlay.

My specs:
Windows 7 32-bit
AMD Phenom II X3 720
Radeon HD4890 Vapor-X GPU
4GB RAM
Razer copperhead mouse

All drivers are up-to-date.