Windows 7: Screen power saving causes the screen to turn on?

I've got a very weird problem.

I've turned on energy saving settings and have the screen turn off after 10 minutes (after 5 minutes the regular screen saver is turned on).



Now... this works fine. Except, sometimes, I come back to my PC and find, instead of either the screen saver or the screen in a power-saving mode... a black screen with the mouse pointer in the middle (screen turned on, obviously). If I move the mouse, the computer works as expected - I get thrown back to the desktop and / or whatever else I was doing.

The issue isn't serious, but I would like to make use of the power-saving feature and... well... save some power. I can manually turn the screen off (and I'm doing this ATM when I'm not at my PC), but there are moments where I simply forget (or I think I won't be away from my PC for long).

Any ideas how to tackle this strange problem?

PS: This whole mess seems to have started after I got a new graphics card. The old one was locking the system up in games. Since I had no graphics card when my old card was sent on warranty, I used the feature of new Intel CPUs, and had my i5 act as the GPU for the time being. After a few weeks I got a new graphics card (same model: AMD Radeon HD 7800 Series) which I installed without a fuss. I've even reinstalled the drivers and anything else video-related. Of course I would like to stress that the problem SEEMS to have started after that. I may be wrong in this aspect.