There are hundreds of posts about this issue. For most you have to wait a long time for it to come back. If it doesn't look at manually installing drivers for motherboard, audio, video, etc.
I have windows 7 64-bit. I used to use xp 64-bit. I have a problem with the game Unreal tournament 3. Whenever i alt-tab out of it, it stops responding and i can't get it to respond (Tried changing priority). The only way i can close it is to use task manager and end process. This didn't happen when i had xp. What's going on?