You're not moving the cursor down to the bottom right-hand corner of your screen, by any chance, are you Trt? If so (in Windows 7 at least) that's meant to display your desktop.
It would still be useful to know:
(a) which version of Windows you're using ;
(b) whether the problem only occurs when you move your cursor to a specific location, or whether it happens at random ; and
(c) whether the problem only occurs when using your web browser (if so, which browser?) or whether it occurs when using other software (such as Word).
Agreed but people who use Macs (or who install Linux) usually mention their O/S in the question. Otherwise (if it's a laptop) it's usually a fairly safe bet that it's some version of Windows!