Update your graphics drivers, or replace your video card. Most (all?) video cards use something called a hardware mouse cursor. Windows doesn't actually draw the mouse cursor on your screen, it just tells the video card what it looks like and where it is. The video card can then bitblt the cursor into the final image at the proper location just before it's displayed. This is much faster and requires less computation than doing it in software.
It sounds like your video card is failing to do this when you alt-tab out of a game. Coupled with the graphics glitches you're seeing, that suggests you have a problem with the video card. So try updating video drivers. If you already have the latest drivers, try completely uninstalling them, rebooting, and reinstalling them. If that doesn't fix it, your video hardware may be faulty. If this only happens during/right after intensive gaming, have you tried measuring its temperature while gaming?