I'm thinking your ATI Mobility Radeon 4100 has problems with alt tabbing back into Blockland, especially with fullscreen.
Temporary fix: Disable fullscreen, and just set it to a high resolution.
You also may need to update your drivers, which I found for you
here.
Download, install, restart. Try it in fullscreen alt tabbing. If the problem persists I'm guessing your chip either can't handle alt tabbing fullscreen or its something else.