I used to have this problem too, only after playing a full screen directx game. When I would return to the desktop, only occasionally, my cursor would "go behind" as I came to think of it.
I noticed that occasionally going back into and out of the game would restore the cursor. Sometimes, just changing my resolution would fix the problem. The only surefire way of fixing it was a reboot, no pet hamster required.
Since then I have got a new system, with a new graphics card, and have yet to see the problem. My old graphics card was a Geforce 440mx, my new one is a Geforce 6600GT. My general workaround when I was experiencing the problem, and didn't want to reboot, was to have a "toggle cursorXP" shortcut easily available (basically just a link to CursorXP.exe) which I would use if I wanted to use rainlendar or widgets.